In vitro response of large colon arterial and venous rings to vasodilating drugs in horses.

OBJECTIVE: To determine in vitro vasomotor response of equine large colon arterial and venous rings with and without endothelium to vasodilator drugs, including dopamine (DOP), dopexamine (DPX), acepromazine (ACE), isoxsuprine (ISX), and nifedipine (NFP). ANIMALS: 7 adult horses. PROCEDURE: Relaxation of large colon arteries and veins in response to vasodilating drugs was determined by measuring the change in tension of vessel rings when exposed to a cumulative concentration range (10(-8) to 10(-4)M) of each drug. Vessel rings, with and without endothelium, were mounted in organ baths, attached to a transducer, and contracted with norepinephrine (NE). Cumulative concentration-response relationships, percentage maximal relaxation, and EC50 (concentration of drug required to relax the NE-induced contracted tissue to 50% of its contracted state) values were calculated. RESULTS: There were significant differences among drugs for EC50 (ACE = ISX < NFP) and percentage maximal relaxation (ACE = ISX > NFP = DPX > DOP) values in veins. Endothelium removal from veins had no significant effect. There were no differences in EC50 values for arteries; however, percentage maximal relaxation was significantly different among drugs (ACE = ISX = NFP > DPX = DOP). Endothelial removal resulted in higher EC50 and lower percentage maximal relaxation values, compared with endothelium-intact arteries. CONCLUSION AND CLINICAL RELEVANCE: ACE and ISX were the most potent and efficacious drugs evaluated and could potentially be used to improve blood flow after correction of large-colon volvulus. Dopamine cannot be recommended because of its biphasic response and potential to further decrease blood flow. Endothelium removal altered the vasodilatory responses of colonic arterial rings, but did not affect venous rings.

Responses of guinea-pig lung parenchymal strips to tracheobronchial lavage fluid from horses affected with summer pasture-associated obstructive pulmonary disease.

The response of parenchymal strips from guinea-pig lungs to tracheobronchial lavage fluid (TBLF) collected from 8 normal horses and from 8 affected with summer pasture-associated obstructive pulmonary disease (SPAOPD) was determined. TBLF was collected during the summer (July) and winter (February) seasons. The serum/TBLF urea nitrogen ratio was used to standardize the mediator concentration in the TBLF. Four strips were used from each guinea-pig. The first strip did not receive any antagonist and served as the control. The second, third and fourth strips received antagonists of PGE2, LTD4 and PAF, respectively at 10(-6) mol/L for 30 min. The tissues were then precontracted with a dose of histamine (10(-5) mol/L) and their responses to 1 ml of TBLF were determined. The study showed that TBLF obtained in the summer from unaffected horses produced a significantly greater relaxation than that from the affected horses, whereas TBLF obtained in the winter from unaffected or affected horses did not cause a significantly different degree of relaxation. Among the antagonist-treated strips, only those exposed to the PGE2 blocker showed a significant reduction in the relaxation caused by TBLF obtained in the summer from SPAOPD horses. This suggests that PGE2 is an important mediator present in the summer in the TBLF from horses affected with SPAOPD.

Involvement of nitric oxide in the mediation of NANC inhibitory neurotransmission of guinea-pig trachea.

1. The involvement of nitric oxide (NO) in the non-adrenergic non-cholinergic inhibitory (NANC-i) neurotransmission was evaluated in guinea-pigs anaesthetized with chloralose-urethane, using a tracheal pouch preparation. 2. The tracheal pouch, a surgically isolated segment of trachea with intact nerve and blood supply, is an in situ method to demonstrate NANC-i response after complete cholinergic and adrenergic blockade using atropine (5 mg kg(-1)) and propranolol (1 mg kg(-1)), respectively. Cervical vagi and sympathetic trunks were isolated and cut cranially. The distal ends of the vagi were positioned on bipolar electrodes for subsequent stimulation with 5 V pulses for 2 ms duration at 15 Hz for a total of 90 s. The relaxation response was measured as a pressure drop (cm of H2O) in the pouch. Each experimental group was composed of six animals. 3. NANC-i responses to two consecutive nerve stimulations at 25 min apart were reproducible. 4. Pouch relaxation responses to electrical nerve stimulations were determined before and after incubation of the pouch with N(omega)-nitro-L-arginine methyl ester (L-NAME; 10(-5) M), a NO synthase (NOS) inhibitor, for 30 min. L-NAME significantly, but not completely, inhibited the NANC-i response of the pouch, suggesting involvement of NO in the NANC-i neurotransmission. 5. The pouch relaxations to vagal stimulations were inhibited significantly after incubation with oxyHb indicating that NO was released. 6. The amount of methaemoglobin (metHb) formed from oxyhaemoglobin (oxyHb) during vagal stimulation was measured by spectrophotometry. Comparison of the values between the control and after nerve stimulation indicated a trend (P = 0.07) toward greater metHb formation in the pouch perfusate after nerve stimulation. 7. NANC-i responses were not significantly inhibited by incubation of the pouch with either of the guanylate cyclase inhibitors, methylene blue or 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ). However, a trend toward significance (P < or = 0.07) was observed. 8. This study demonstrated that NO is involved in NANC-i neurotransmission. However, the findings did not conclusively support the contention that NO is the sole neurotransmitter of NANC inhibition. It is possible that NO produced relaxation of guinea-pig trachea through a cGMP-independent mechanism.

Biphasic responses of equine colonic vessel rings to vasoactive inflammatory mediators.

1. The role of endothelium in modulating equine colonic vessel responses to histamine (HST), 5-hydroxytryptamine (5-HT), bradykinin (BK) and acetylcholine (ACh) was evaluated in vitro. 2. Segments of mesenteric arteries and veins were collected from the left ventral colon of six adult horses destined for euthanasia for reasons unrelated to cardiovascular or gastrointestinal systems. Vessels were gently cleansed and cut into 4 mm wide rings. 3. Three vessel conditions namely endothelium intact, endothelium removed and N omega-nitro-L-arginine methyl ester (L-NAME)-treated were used for both arterial and venous rings. Each ring was placed in an organ bath with oxygenated Tyrode's solution. One side of the ring was fixed to the floor of the bath and the other side to a force-displacement transducer interfaced with a polygraph. 4. An initial tension of 2 g was applied to rings which were allowed to equilibrate for 45 min. The bath solution was gently replaced every 15 min and tension was readjusted to 2 g each time except following the last wash. 5. Rings were precontracted with a single EC25 dose of noradrenaline and after the response plateaued, cumulative concentration (10(-12)-10(-4) M) response curves were determined for each agent on separate rings. The relaxation from the precontracted level to the baseline was considered as 100% relaxation. Maximal relaxation and maximal contractions were statistically analyzed. 6. All agents induced a relaxation response initially, followed by a contractile phase as the concentrations increased in both arteries and veins, thus, making a biphasic concentration-response curve. In arteries, relaxation produced by ACh was significantly greater than 5-HT. Endothelium removal and L-NAME treatment significantly reduced relaxation in arteries. Only endothelium removal produced a significant reduction of relaxation in veins. 7. In both arteries and veins, HST and 5-HT produced significantly greater contraction than ACh or BK. No significant change in contraction was observed in arteries either by endothelium removal or L-NAME treatment, however, contraction was significantly reduced in veins by endothelium removal. 8. These findings suggest that the endothelium plays a major role in modulating equine colonic arterial relaxation via nitric oxide and venous contraction via endothelium-derived contractile mediators, probably endothelin and/or arachidonates.

Sedative and analgesic effects of medetomidine in beagle dogs infected and uninfected with heartworm.

The sedative and analgesic effects of medetomidine were evaluated in heartworm-infected (HW+) and uninfected (HW-) beagle dogs by intravenous (IV) and intramuscular (IM) administration of 30 micrograms/kg and 40 micrograms/kg doses, respectively. Posture, response to noise and the pedal reflex were monitored. A procedure for mock radiographic positioning was performed to evaluate its overall clinical use. Observation times were 0, 15, 30, 60, 90, 120 and 180 min. In addition, the times from injection until the dog could not stand on its feet (down time), from lateral to sternal recumbency (sternal recumbency time), and from sternal recumbency to rising again (rising time) were also noted. Medetomidine produced rapid sedation and analgesia by both routes. Down times for the IM and IV routes were similar, which verified the manufacturer's recommended doses. The HW+ dogs had shorter down times, probably owing to increased blood flow to the brain caused by adrenergic alpha-2 activity. Sternal recumbency and rising times did not differ between the groups, suggesting a similar metabolism. Sedation and analgesia were adequate for performing the procedure in all dogs. HW- dogs showed less resistance to handling during the procedure than HW+ dogs. Overall, medetomidine seems to be a suitable agent for short-term chemical restraint in dogs, even with subclinical heartworm infestation.

Bioavailability and biotransformation of benzo(a)pyrene in an isolated perfused In situ catfish intestinal preparation.

In the aquatic environment, diet is an important route of exposure for the common contaminant and procarcinogen benzo(a)pyrene (BaP). Dietary organisms vary in their BaP content and in contaminated areas often contain other xenobiotics including cytochrome P4501A inducers. This study examined the effect of dose and previous dietary exposure to the inducer ss-naphthoflavone (BNF) upon the intestinal metabolism of BaP and the systemic bioavailability of BaP-derived products in catfish. BaP was administered at 2 and 20 microM into in situ-isolated perfused intestines of control and BNF-pretreated catfish. The intestine formed an array of metabolites in all treatments including potentially hazardous metabolites such as BaP-7,8 and 9,10 dihydrodiols and 6-methyl-BaP. BNF treatment disproportionally increased the contribution of BaP-7,8 and 9,10 dihydrodiols relative to the contributions of other metabolites. A greater percentage of metabolites was evident as conjugates in 2 microM controls, whereas a greater percentage of unconjugated metabolites was evident for 20 microM controls and BNF treatments of both dosages. BNF pretreatment and the higher 20 microM BaP dosage resulted in greater bioavailability, with 2.6-5.5-fold and 3.0-6. 3-fold increases in systemically available BaP products, respectively. Metabolites represented 10.2-23.1% of the increased bioavailability with BNF treatment, suggesting that mechanisms, in addition to induced metabolism, may be operative. These results indicate that intestinal bioavailability, level of biotransformation, and the metabolic profile of BaP-derived products entering the blood from the intestine may be altered by dose and dietary BNF pretreatment.

Role of endothelium and nitric oxide in the in vitro response of equine colonic venous rings to vasoconstrictor agents.

OBJECTIVE: To determine in vitro contractile responses of equine colonic veins to various vasoconstrictor agents. ANIMALS: Colonic veins collected from 8 adult horses. PROCEDURE: Veins were cut into 4-mm-wide rings, placed in organ baths at 37 C, and attached to a force-transducer interfaced with a polygraph; 2 g of tension was applied, and rings were allowed to equilibrate for 45 minutes. Bath solution was replaced, and tension was reapplied at 15-minute intervals. Cumulative concentration responses (10(-8) to 10(-4) M) were determined for each agent, using separate rings (n = 8). Three vein groups were evaluated: endothelium-intact, endothelium-denuded, and N omega-nitro-L-arginine methyl ester (L-NAME, 10(-5) M)-treated. Maximal responses by each vein to each agent were considered 100%; responses to lower concentrations were calculated as percentage of maximum. RESULTS: Considering all vein groups, comparison of the doses that caused 50% of the maximal contraction revealed relative sensitivity of colonic veins to be: angiotensin II (ANG) > thromboxane B2 analogue (TXB) > 5-hydoxytryptamine (5HT) > norepinephrine (NE) > histamine (HST) > prostaglandin F2 alpha (PGF) > vasopressin (VP). Compared with ANG, PGF, TXB, and VP, treatment with HST, 5HT, and NE evoked significantly greater responses. Endothelium-denuded and L-NAME-treated colonic veins had significantly greater maximal contractile responses than did endothelium-intact veins. CONCLUSIONS: Response of colonic veins to vasoconstrictor agents was differential; sensitivity was not altered by endothelium removal or L-NAME treatment; maximal responses of endothelium-intact veins were greater than those of endothelium-denuded and L-NAME-treated veins; and responses of endothelium-denuded and L-NAME-treated veins were not different. CLINICAL RELEVANCE: Alterations in colonic veins that mimic conditions associated with large-colon volvulus may contribute to blood flow alterations, edema formation, and vascular responses to hypovolemic and endotoxic shock.

Role of endothelium and nitric oxide in the response of equine colonic arterial rings to vasoconstrictor agents.

OBJECTIVE: To determine the in vitro contractile responses of equine colonic arteries to angiotensin II, histamine, serotonin, norepinephrine, prostaglandin F2 alpha, vasopressin, and a thromboxane-B2-analogue. STUDY DESIGN: The tension generated in colonic arterial rings placed in organ baths with oxygenated Tyrode's solution at 37 degrees C after exposure to the previously mentioned chemical agents was measured using force-transducers interfaced with a polygraph. SAMPLE POPULATION: Large colon arterial rings collected from eight horses. METHODS: The rings were allowed to equilibrate for 45 minutes after applying 2 g tension. Bath solution was replaced and tension reapplied at 15-minute intervals. Cumulative-concentration-responses were determined for concentrations ranging from 10(-8) M to 10(-4) M on three vessel groups namely endothelium intact, endothelium denuded, and L-NAME treated. The maximal response for each vessel was considered as 100%; responses to lower concentrations were calculated as a percentage of the maximum. The EC50 value was determined for each concentration-response relationship of each agent. RESULTS: Vessels with denuded endothelium or those incubated with L-NAME had greater contractile responses. Angiotensin, histamine, serotonin, and norepinephrine produced greater maximal responses than the other agents. Endothelium denuded rings had lower EC50 values. Responses to norepinephrine and serotonin were affected less by denudation. CONCLUSIONS: Endothelium plays an important role in modulating responses of colonic arterial rings to contractile agents. Endothelium-derived vasodilators, other than nitric oxide, may modulate contractile responses of equine colonic arteries. CLINICAL RELEVANCE: Endothelial damage associated with colonic vovulus may be a major factor for sustained reduced perfusion after surgical correction.

Large bowel transit times using radiopaque markers in normal cats.

Seven normal, young adult, female cats each had its colon evacuated with warm-water enemas (at 24 hours, 12 hours, and one hour) before oral administration of a gelatin capsule containing 20 radiopaque markers to determine normal colonic transit times. Abdominal radiographs were taken twice daily until all markers were expelled. There were no significant differences between animals or trials (p = 0.05 or less). Markers accumulated in the first section of large bowel within eight hours. Evacuation from the colon took another 40 hours. Cecum (right side midline), left side midline, and rectum were defined radiographically. This study establishes large bowel transit times in normal cats and provides the basis for similar evaluation in constipated cats.

Evidence for serotonin involvement in the NANC excitatory neurotransmission in the catfish intestine.

1. Four putative neurotransmitters (serotonin, substance P, ATP (alpha-beta-methylene-ATP), and vasoactive intestinal peptide, VIP) of the non-adrenergic non-cholinergic (NANC) innervation were examined for their role in the NANC excitatory neurotransmission in channel catfish intestine after adrenergic and cholinergic blockade. 2. VIP at concentrations ranging from 10(-12)M to 10(-4)M did not produce either a relaxant or a contractile response in these segments. 3. Serotonin, substance P and alpha-beta-methyl-ATP produced contractile responses in a dose-dependent manner. Their EC50 values were 5 x 10(-7)M, 5 x 10(-9)M and 5 x 10(-9)M and 5 x 10(-6)M, respectively. 4. Electrical field stimulation of the intestinal segments produced a predominant excitatory response after complete blockade of adrenergic and cholinergic divisions, suggesting a predominant NANC excitatory innervation. 5. Three types of serotonin receptor antagonists, namely methiothepin (predominantly a 5-HT1 antagonist), ketanserin (a selective 5-HT2 antagonist), methysergide and cyproheptadine (predominantly 5-HT2 blockers) and metoclopramide (a selective 5-HT3 blocker) were tested for their effectiveness against serotonin and EFS-induced contractions. Methiothepin, methysergide, cyproheptadine and metoclopramide produced significant blockade of the response to serotonin, whereas only methiothepin and cyproheptadine produced blockade of EFS-induced response. 6. Three agents tested for substance P blockade, namely spantide, 4-11 fragment of substance P, and methysergide (also a serotonin blocker), did not produce significant inhibition of the response to either substance P or EFS. 7. Suramin at a dose that blocked the ED50 concentration of ATP did not produce a significant blockade of the response to EFS suggesting that ATP-involvement in the NANC-e neurotransmission is unlikely. 8. This study confirmed the involvement of serotonin in the expression of non-adrenergic non-cholinergic excitatory response of the channel catfish intestine.

Cardiopulmonary effects of medetomidine in heartworm-infected and noninfected dogs.

Medetomidine, an investigational drug indicated for clinical use as a short-term chemical restraint in dogs, was evaluated for its cardiopulmonary effects, in 10 naturally heartworm-infected (HW+) and 10 noninfected (HW-) Beagles. The drug was randomly administered i.v. (30 micrograms/kg of body weight) and i.m. (40 micrograms/kg) in single injections to all dogs. Heart rate, respiratory rate, ECG, blood gas tensions, blood pH, central venous and arterial pressures were measured at 0, 15, 30, 60, 90, 120, and 180 minutes. Medetomidine induced an immediate significant (P < or = 0.001) increase in mean arterial blood pressure followed by decreased blood pressure that remained below normal throughout the study in both groups, irrespective of route of administration. Medetomidine increased central venous pressure, over time, for both groups and both routes of administration. Heart and respiratory rates were significantly (P < or = 0.001) decreased after medetomidine administration and remained reduced for the duration of the study in all dogs. The ECG variables were not significantly different between groups or between routes of administration. The HW+ dogs tended to have higher mean PaO2 than did HW- dogs at several postinjection determination times, particularly when the drug was administered i.m. The PaO2 decreased during the first 30 minutes in both groups and tended to increase gradually thereafter. The pH decreased over time for both groups and both routes. A significant (P < or = 0.05) decrease in pH was seen in the HW- dogs, compared with HW+ dogs at each measuring time for both routes.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-adrenergic non-cholinergic (NANC) excitatory response of the channel catfish intestine.

1. Optimal parameters for electrical field stimulation (EFS) of catfish pyloric and middle intestinal segments were determined (15 Hz, 60 V) from a range of frequencies (5-45 Hz) and voltages (40-120 V) using a modified Magnus' method. Contractile responses were produced by EFS which were reproducible and showed no significant difference between the tissues. 2. The contractile cholinergic responses of the tissues to carbachol and acetylcholine (ACh) were blocked by atropine on an equimolar concentration, whereas, these responses were enhanced in the presence of neostigmine, and acetylcholinesterase inhibitor. 3. Adrenergic responses were examined with noradrenaline (NA). NA produced contraction of the segments only, at a concentration of 10(-4) M. Among the various adrenoceptors, beta-adrenoceptor stimulation produced a weak relaxation whereas, both alpha 1- and alpha 2-adrenoceptor stimulation produced contractions, of which alpha 2-induced contraction was of greater magnitude. The beta, alpha 1 and alpha 2 responses were blocked by their respective blocking agents propranolol, prazosin and yohimbine. 4. The autonomic components of the response to EFS were determined by using selected cholinergic and adrenergic antagonists separately or collectively. Cholinergic blockade with atropine did not produce a significant blockade of the EFS-induced response. Similarly, blockade of beta-adrenoceptors with propranolol did not modulate the contractile response to EFS to any significant level. Blockade by prazosin or yohimbine did not significantly change the contractile response to EFS. After a complete blockade of the adrenergic and cholinergic divisions, the intestinal segments still showed a contractile response to EFS which was not significantly different from the control response. This indicated the presence of a non-adrenergic non-cholinergic (NANC) response. 5. Tetrodotoxin, at 10(-6) M, significantly blocked the EFS-induced NANC response suggesting a neurogenic origin for the response. 6. The present study indicated that the EFS-induced response of the catfish intestinal segments is predominantly NANC-e in nature suggesting an important role for it in the regulation of intestinal motility.

Blockade of electrical field stimulation-induced nonadrenergic noncholinergic (NANC) excitatory response of the guinea pig ileum.

Determination of guinea pig ileal response to electrical field stimulation (EFS) before and after complete adrenergic and cholinergic blockade showed that the nonadrenergic noncholinergic excitatory (NANC-e) response contributed 40% to the total contractile response. NANC-e responses were reproducible to consecutive identical EFS. Tetrodotoxin significantly blocked the NANC-e response. The NANC-e responses obtained before and after treatment of the tissues with receptor antagonists of histamine (pyrilamine), substance P ([D-Arg1,D-Pro2,D-Trp7,9,Leu11]-SP) and serotonin (5-HT1 & 5-HT2) indicated that the response could be blocked by substance P and serotonin antagonists. Only the serotonin antagonist cyproheptadine produced a dose-dependent blockade. Both substance P and serotonin are suggested to play an important role in the NANC-e neurotransmission.

Stimulation frequency-dependent nonadrenergic noncholinergic airway responses of the guinea pig.

We examined the inhibitory and excitatory components of the nonadrenergic noncholinergic (NANC) innervation of the guinea pig airways by in vivo and in vitro methods. Electrical stimulation of the vagus in chloralose-urethan-anesthetized guinea pigs after cholinergic and adrenergic blockade produced peripheral airway constriction (insufflation pressure) and tracheal relaxation (pouch pressure). Vagal stimulation was applied for 90 s at 5-V pulses of 2-ms duration at frequencies of 5, 15, 25, and 35 Hz in each group (n = 6). The pouch relaxation peaked at 15 Hz. The insufflation pressure was highest at 5 Hz. Field stimulations of the same frequencies were applied on tracheal spirals and lung parenchymal strips. The maximal relaxation of the trachea occurred at 15-35 Hz. The lung parenchymal strip tensions increased almost linearly as the frequency increased from 5 to 35 Hz. The results of the study indicated a frequency-dependent response for both excitatory and inhibitory components of the NANC, which operate at different frequencies for optimal responses.

Epithelial influence on the tracheal nonadrenergic inhibitory response.

1. The role of epithelium in the non-adrenergic non-cholinergic (NANC) inhibitory response to electrical nerve stimulation as well as to the putative neurotransmitter, vasoactive intestinal polypeptide (VIP), was evaluated in guinea-pigs anaesthetized with chloralose-urethane. 2. The tracheal pouch, an in vivo method to demonstrate the NANC inhibitory response, was used in all experiments. The relaxation response was measured as a pressure drop (cm of H2O) in the pouch. The animals were given atropine (5 mg kg-1) and propranolol (1 mg kg-1) intraperitoneally to block adrenergic and cholinergic responses in the pouch. Cervical vagi were isolated and cut craneally. The distal ends were positioned on bipolar electrodes for subsequent stimulation with 5V pulses of 2 ms duration at 15 Hz for a total of 90 s. 3. The reproducibility of NANC responses to two consecutive nerve stimulations at 20 min apart was determined in group 1. 4. In group 2 pouch relaxations to electrical nerve stimulations were determined before and after complete epithelial denudation (determined by histological and pharmacological methods) of the pouch. 5. To determine the influence of relaxant prostaglandins synthesized by the epithelial cells, the effect of indomethacin (given either intravenously or into the pouch) on the pouch relaxation due to NANC stimulation was studied in groups 3 and 4 respectively. 6. In group 5, in order to distinguish between the effects of epithelium removal and prostaglandins, the animals were pretreated with indomethacin (i.v.) 30 min before the experiment. The pouch relaxation to nerve stimulation was then determined before and after the removal of epithelium. 7. The reproducibility of the pouch relaxations to consecutive single doses of VIP 10(-9) M at 20 min apart was determined in group 6. 8. In group 7, the pouch relaxation to a single dose of VIP was determined before and after intravenous indomethacin administration, but with the pouch epithelium left intact. 9. The effect of epithelium removal on VIP-induced pouch relaxation was determined in group 8. VIP-induced response was determined before and after the epithelium removal. 10. The study showed that both epithelium removal and indomethacin administration independently had significant effects on the decrease of the pouch relaxation (NANC inhibitory response). However, epithelium removal significantly diminished pouch relaxation despite indomethacin pretreatment suggesting a second, non-arachidonic dependent, mechanism (possibly via a relaxing factor) whereby epithelium maintains airway homeostasis.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for vasoactive intestinal peptide as a mediator of non-adrenergic non-cholinergic neurotransmission in the trachea.

1. The tracheal pouch, a surgical preparation designed to demonstrate non-adrenergic non-cholinergic inhibition, was prepared in chloralose/urethane-anaesthetized and positively ventilated guinea-pigs. The animals were given atropine and propranolol intraperitoneally to block cholinergic and adrenergic divisions of the autonomic innervation. The cervical vagi and sympathetic trunks were isolated bilaterally and cut cranially. 2. The relaxation responses of the pouch to graded concentrations of VIP (10(-11) M to 10(-6) M) were determined. Two consecutive dose-response curves at 20 min apart were determined in the control group. The VIP dose-response curves in the control group were reproducible and failed to show statistical significance upon paired Student's t-test. 3. [Ac-Tyr1,D-Phe2]-GRF(1-29)-amide, a VIP antagonist hereby referred to as antagonist-1 was tested for its ability to inhibit the VIP-induced pouch relaxation. Separate groups of animals were used for each concentration (10(-8) M, 10(-6) M or 10(-5) M) of the antagonist. VIP dose-response curves were determined before and after the pouch was incubated with the antagonist for 10 min. The second curve was determined after rinsing the pouch gently with saline and allowing 5 min for the pouch to stabilize. Statistical analysis showed that only 10(-5) M of the antagonist significantly blocked the VIP-induced tracheal pouch relaxation. 4. [4-C1-D-Phe6,Leu17]-VIP, another VIP antagonist hereby referred to as antagonist-2 was tested similarly for its ability to block the VIP-induced pouch relaxation. The significant blockade of the VIP-induced pouch relaxation was obtained with this antagonist at a concentration of 10(-6) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of arrhythmogenic doses of epinephrine in heartworm-infected and noninfected dogs.

The arrhythmogenic dose of epinephrine (ADE) was determined in heartworm-infected and noninfected (control) dogs during thiamylal-induced and halothane-maintained anesthesia to assess the myocardial sensitization. The ADE in heartworm-infected dogs (2.42 +/- 0.26 micrograms/kg of body weight) was significantly lower than that for the controls (3.36 +/- 0.29 micrograms/kg). After 2 weeks, ADE was determined again in these dogs after atropine treatment. Atropine treatment lowered the ADE to 1.76 +/- 0.33 micrograms/kg and 1.77 +/- 0.19 micrograms/kg in heartworm-positive and -negative dogs, respectively. After 2 weeks more, the ADE was determined after administration of prazosin, an alpha 1-antagonist. Only 2 of 6 controls and 3 of 6 heartworm-positive dogs had arrhythmias after a threefold increase of ADE. The mean ADE in the dogs that responded to treatment were 7.4 micrograms/kg and 7.2 micrograms/kg for heartworm-positive and -negative dogs, respectively. The finding of this study indicated that ADE in heartworm-infected dogs were lower than those in the control dogs, which makes the heartworm-infected dogs more vulnerable to arrhythmia during anesthesia. Atropine did not protect the dogs of either group. However, prazosin protected the dogs of both groups by significantly increasing the threshold of the ADE. On the basis of our findings, to reduce the risk of arrhythmia, we suggest that routine screening of dogs for heartworm infection be done before anesthetics are used.

Vasoactive intestinal peptide (VIP), a putative neurotransmitter of nonadrenergic, noncholinergic (NANC) inhibitory innervation and its relevance to therapy.

Nonadrenergic, noncholinergic (NANC) innervation, the third division of the autonomic nervous system, has both inhibitory and excitatory parts. The excitatory part received only limited attention. Substance P has been suggested to be the neurotransmitter of the excitatory part. The NANC-inhibitory innervation has recently been studied in detail. Although the neurotransmitter has not been conclusively identified, a substantial body of evidence exists to support vasoactive intestinal peptide (VIP) as the neurotransmitter. VIP is widely distributed in the body. Reports show that this innervation in animals and man plays a significant role in both health and disease. Pathological conditions could result from either an increase or decrease in VIP production. An absence of VIP-producing neurons has been identified to be responsible for Hirschsprung's disease in the alimentary system and hyperactive airways in the respiratory system. An increase in VIP production is associated with chronic water diarrhea syndrome in humans. Taking these factors into consideration, various therapeutic measures are suggested with the use of VIP or its antagonists.

The functional conversion hypothesis: a contributor to exercise-induced asthma?

Studies on receptor stability suggest that functional conversion of adrenoceptors between alpha and beta can occur in mammalian myocardium due to variations in the environment such as temperature changes, pH or hormonal changes. If adrenoceptors of the respiratory system behave similarly, heat and water loss of airways noted during hyperventilation could lead to functional loss of inhibitory beta and expression of excitatory alpha adrenoceptors. This would have the effect of counter-action of adrenergic homeostatic mechanisms which may be of particular importance in asthmatic subjects. The hypothesis of adrenoceptor interconversion could contribute to the airway obstruction observed during exercise in asthmatics. This concept is supported by scattered reports of the efficacy of alpha adrenergic antagonists in preventing exercise-induced asthma.

Cross-desensitization of VIP- and NANC-mediated inhibition of the guinea-pig tracheal pouch.

1. Chloralose/urethane anaesthetized guinea-pigs were used for preparation of the tracheal pouch, which demonstrates the NANC innervation. Isolated vagi cut cranially were positioned on bipolar electrodes for subsequent stimulation. The animals were given atropine and propranolol intraperitoneally before the experimental procedure to assure that responses would be from non-adrenergic, non-cholinergic components. 2. ATP and VIP, two putative neurotransmitters for non-adrenergic, non-cholinergic (NANC) inhibitory innervation caused relaxation responses of the pouch. Both VIP and ATP (10(-9) M) caused auto-desensitization after four repeated administrations into the pouch. 3. If NANC nerve stimulation causes relaxation of tracheal smooth muscle by release of one of these agents onto a specific receptor, repeated exposure to that agent by nerve stimulation or exogenous application could cause diminished responsiveness from cross-desensitization, yet maintained responsiveness to an agent acting at other receptors. 4. Relaxation responses to vagal stimulation were determined before and after the pouch was incubated for a 10-min period with either ATP or VIP (10(-6) M). ATP incubation did not produce significant (P less than 0.05) change in the degree of relaxation in response to nerve stimulation. In contrast, VIP incubation did cause diminished relaxation of the pouch in response to subsequent nerve stimulation. 5. The ability of the agents at 10(-9) M and 10(-6) M to cause relaxation of the pouch was next examined before and after repeated NANC nerve stimulation. Relaxation of the pouch to ATP was not significantly affected by intervening repeated nerve stimulations. However, relaxation in response to VIP was significantly inhibited by repeated nerve stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)