Design and synthesis of 4-substituted benzamides as potent, selective, and orally bioavailable I(Ks) blockers.

Multiple delayed rectifier potassium currents, including I(Ks), are responsible for the repolarization and termination of the cardiac action potential, and blockers of these currents may be useful as antiarrhythmic agents. Modification of compound 5 produced 19(S) that is the most potent I(Ks) blocker reported to date with >5000-fold selectivity over other cardiac ion channels. Further modification produced 24A with 23% oral bioavailability.

Pharmacologic characterization of BMS-191095, a mitochondrial K(ATP) opener with no peripheral vasodilator or cardiac action potential shortening activity.

Previous work described ATP-sensitive K(+) channel (K(ATP)) openers (e.g., BMS-180448), which retain the cardioprotective activity of agents such as cromakalim while being significantly less potent as vasodilators. In this study, we describe the pharmacologic profile of BMS-191095, which is devoid of peripheral vasodilating activity while retaining glyburide-reversible cardioprotective activity. In isolated rat hearts subjected to 25 min of global ischemia and 30 min of reperfusion, BMS-191095 increased the time to onset of ischemic contracture with an EC(25) of 1.5 microM, which is comparable to 4.7 microM and 3.0 microM for cromakalim and BMS-180448, respectively. Comparisons of cardioprotective and vasorelaxant potencies in vitro and in vivo showed BMS-191095 to be significantly more selective for cardioprotection with virtually no effect on peripheral smooth muscle, whereas cromakalim showed little selectivity. In addition to increasing the time to the onset of contracture, BMS-191095 improved postischemic recovery of function and reduced lactate dehydrogenase release in the isolated rat hearts. The cardioprotective effects of BMS-191095 were abolished by glyburide and sodium 5-hydroxydecanoate (5-HD). BMS-191095 did not shorten action potential duration in normal or hypoxic myocardium within its cardioprotective concentration range nor did it activate sarcolemmal K(ATP) current (< or =30 microM). BMS-191095 opened cardiac mitochondrial K(ATP) with a K(1/2) of 83 nM, and this was abolished by glyburide and 5-HD. These results show that the cardioprotective effects of BMS-191095 are dissociated from peripheral vasodilator and cardiac sarcolemmal K(ATP) activation. Agents like BMS-191095 may owe their cardioprotective selectivity to selective mitochondrial K(ATP) activation.

Identification of [3H]P1075 binding sites and P1075-activated K+ currents in ovine choroid plexus cells.

This study examined the pharmacological characteristics of binding sites for the potent K+ channel opener [3H]P1075, as well as the functional effects of P1075 on ionic currents and membrane potential, in ovine choroid plexus (OCP) cells. [3H]P1075 bound to OCP cells with a Kd of 26 +/- 4 nM and a Bmax of 10400 +/- 480 sites/cell. Labelled sites were stereoselective and inhibited by potassium channel openers with a rank order of potency: P1075 > BMS-182264, ((4-[[9cyanoimino)[(1,2,2-trimethylpropyl)amino]-methyl]amino]benz onitrile) > pinacidil >> nicorandil > diazoxide. The K(ATP) channel antagonist glyburide inhibited [3H]P1075 binding with a Ki of 2 microM. The presence of K(ATP) channels on OCP cells was examined by patch clamp and fluorescent (membrane-potential sensitive dye) techniques. In some cells, P1075 activated an outward potassium current which was blocked by glyburide. P1075 produced a glyburide-sensitive, concentration-dependent, hyperpolarization of OCP cells. Levcromakalim hyperpolarized more strongly than its 3R,4S enantiomer, BRL 38226 ((3R-trans)-3,4-dihydro-3-hydroxy-2,2-dimethyl-4-(2-oxo-1-pyrrolidinyl)- 2H-1-benzopyran-6-carbonitrile) indicating a stereoselective interaction. These data indicate that epithelial OCP cells contain glyburide-sensitive K(ATP) channels.

Cardioprotective effect of diazoxide and its interaction with mitochondrial ATP-sensitive K+ channels. Possible mechanism of cardioprotection.

Previous studies showed a poor correlation between sarcolemmal K+ currents and cardioprotection for ATP-sensitive K+ channel (KATP) openers. Diazoxide is a weak cardiac sarcolemmal KATP opener, but it is a potent opener of mitochondrial KATP, making it a useful tool for determining the importance of this mitochondrial site. In reconstituted bovine heart KATP, diazoxide opened mitochondrial KATP with a K1/2 of 0.8 mumol/L while being 1000-fold less potent at opening sarcolemmal KATP. To compare cardioprotective potency, diazoxide or cromakalim was given to isolated rat hearts subjected to 25 minutes of global ischemia and 30 minutes of reperfusion. Diazoxide and cromakalim increased the time to onset of contracture with a similar potency (EC25, 11.0 and 8.8 mumol/L, respectively) and improved postischemic functional recovery in a glibenclamide (glyburide)-reversible manner. In addition, sodium 5-hydroxydecanoic acid completely abolished the protective effect of diazoxide. While-myocyte studies showed that diazoxide was significantly less potent than cromakalim in increasing sarcolemmal K+ currents. Diazoxide shortened ischemic action potential duration significantly less than cromakalim at equicardioprotective concentrations. We also determined the effects of cromakalim and diazoxide on reconstituted rat mitochondrial cardiac KATP activity. Cromakalim and diazoxide were both potent activators of K+ flux in this preparation (K1/2 values, 1.1 +/- 0.1 and 0.49 +/- 0.05 mumol/L, respectively). Both glibenclamide and sodium 5-hydroxydecanoic acid inhibited K+ flux through the diazoxide-opened mitochondrial KATP. The profile of activity of diazoxide (and perhaps KATP openers in general) suggests that they protect ischemic hearts in a manner that is consistent with an interaction with mitochondrial KATP.

Alterations in Ito1, IKr and Ik1 density in the BIO TO-2 strain of syrian myopathic hamsters.

The BIO TO-2 strain of cardiomyopathic hamster provides a model of dilated low output heart failure. The goal of the study was to determine whether changes in potassium currents occur in this model of heart failure. The densities of Ito1, IKr and IK1 in 8-month-old myopathic hamsters were not significantly different from their age-matched controls. The half-maximum activation voltage (V1/2) of IKr and Ito1, as well as the voltage-dependence of Ito1 inactivation were also similar in both groups at 8 months. Ito1 inactivation exhibited a double exponential time-course; the slow component (tau2), but not the rapid component (tau1), was larger in the myopathic animals. The densities of Ito1, IKr and IK1 were not significantly different in the 8- and 10-month-old control animals. However, the densities of Ito1, IKr and IK1 were all significantly lower in the 10-month-old myopathic hamsters relative to the 10-month-old controls. The V1/2 for IKr and Ito1 activation was the same in myopathic and control animals. tau2, but not tau1, of Ito1 inactivation was again larger in the myopathic animals. The voltage-dependence of Ito1 inactivation was shifted slightly, but significantly, positive in the myopathic animals. Lastly, a sustained outwardly rectifying current that activated upon depolarization was found to be larger in the myopathic animals at both 8 and 10 months of age. In conclusion, many of the alterations in potassium current densities in the 10-month-old cardiomyopathic hamsters are qualitatively similar to the changes observed in the failing human heart.

The novel cardioprotective agent BMS-180448 activates a potassium conductance in cardiac and vascular smooth muscle.

The goal of the present study was to further characterize the effects of the novel cardioprotective agent BMS-180448 on potassium fluxes in cardiac and vascular smooth muscle. Exposure of voltage-clamped guinea pig ventricular myocytes to BMS-180448 (300 microM) produced an inhibition of IK followed by the delayed (5.5 +/- 0.5 min) activation of a large time-independent potassium current. At 100 microM, BMS-180448 produced only inhibition of IK. The BMS-180448 activated current was refractory to block by 30 microM glyburide but was largely inhibited by 100 microM alinidine (84 +/- 6% inhibition at +40 mV). Cromakalim (100 microM)-activated currents were fully inhibited by 3 microM glyburide and 79 +/- 4% blocked by 100 microM alinidine. The current responses to BMS-180448 were unaffected by the inclusion of 10 mM UDP (100 microM ATP) in the pipette. BMS-180448 also produced a concentration-dependent increase in 86Rb efflux from aortic strips; efflux responses were increased in low calcium medium and fully antagonized by 3 microM glyburide. Thus, BMS-180448 activates a potassium conductance in both cardiac and smooth muscle. The glyburide sensitivity of the BMS-180448-induced increase in 86Rb efflux from the aortic preparations suggests that this drug activates IKATP in vascular smooth muscle. Moreover, the observation that BMS-180448 (100 microM) partially inhibits the effects of cromakalim in ventricular muscle cells suggests that these drugs interact, directly or indirectly, with a common site in cardiac muscle.

Pharmacological characterization of the isolated canine prostate.

PURPOSE: The goal of the present study was to characterize the responses of the isolated normal canine prostate to various contracting and relaxing stimuli to determine which pharmacological agents may have utility against the dynamic component of benign prostatic hyperplasia (BPH). MATERIALS AND METHODS: Isometric force development was measured in isolated strips of prostate tissue. RESULTS: The alpha-adrenergic agonists were the most efficacious stimulants tested (phenylephrine EC50=2.1 microM.). Endothelin-1, acting primarily via ETA receptors, was more potent (EC50=27nM.) but less efficacious. Histamine (EC50=14.7 microM.), serotonin (EC50=0.12 microM.), carbachol (EC50=5.9 microM.) and KC1 (EC50=48.8 mM.) were also less efficacious than phenylephrine. Nifedipine was a potent (IC50=28 nM.) and efficacious (74% inhibition) inhibitor of phenylephrine-induced force. Potassium channel activator drugs were also efficacious relaxants, producing approximately 80% inhibition of force; rank order of potency was P1075 > cromakalim > diazoxide. Sodium nitroprusside was a weak relaxant, producing only approximately 40% relaxation at a concentration of 100 micronM. Both isoproterenol and forskolin were effective relaxants (75 to 90% relaxation). CONCLUSIONS: We conclude that potassium channel activators, adenylate cyclase stimulators, or endothelin antagonists may have utility against the dynamic component of outflow obstruction secondary to BPH.

A comparison between the effects of BMS-180448, a novel K+ channel opener, and cromakalim in rat and dog.

BMS-180448 [(3S-trans)-N-(4-chlorophenyl)-N'-cyano-N"-(6-cyano-3, 4-dihydro-3-hydroxy-2,2-dimethyl-2H-1-benzopyran-4-yl) guanidine] is a structural analog of cromakalim, which was found to similarly decrease ischemic injury, but was 18- to 100-fold less potent as a vasodilator. In the present study, the vascular and cardiac effects of cromakalim and BMS-180448 were evaluated in both in vitro and in vivo preparations. Cromakalim evoked a concentration-dependent relaxation to a K(+)-induced contracture in rat aorta. BMS-180448 behaved in a similar fashion but was 18-fold less potent than cromakalim. Measurements of ischemic damage made in isolated perfused rat hearts demonstrated that cromakalim and BMS-180448 were equipotent as cardioprotective agents; time to contracture was increased with an EC25 value of 4.8 and 4.7 microM, respectively, and lactate dehydrogenase levels were significantly reduced compared to those in the presence of vehicle. In vivo electrophysiologic studies in anesthetized dogs were conducted at basic cycle lengths of 400, 333, and 286 ms, and showed that BMS-180448 caused no significant effect on electrophysiologic parameters with the exception of decreasing atrial effective refractory periods by 12 +/- 3% and 17 +/- 4% at 3 and 10 mg/kg, respectively. There was also a significant drop in mean blood pressure of 18 +/- 5% and 33 +/- 4% at these doses. In contrast, cromakalim was shown to produce shortening of atrial to His conduction time (20 +/- 7%; basic cycle length = 286 ms), atrial effective refractory period (34 +/- 3%; basic cycle length = 400 ms), ventricular effective refractory period (14 +/- 2%; basic cycle length = 400 ms), wavelength (13 +/- 3%; basic cycle length = 400 ms), PR-interval (14 +/- 3%; basic cycle length = 333 ms) and mean blood pressure (65 +/- 3%; basic cycle length = 400 ms) at a dose of 0.3 mg/kg. No supraventricular or ventricular arrhythmias were observed for either compound tested. Based on the reduced cardiac electrophysiologic and vascular effects of BMS-180448, we suggest that BMS-180448 should provide cardioprotective efficacy similar to cromakalim with reduced risk of hypotension or arrhythmias.

Functional role of endothelin ETA and ETB receptors in venous and arterial smooth muscle.

The functional importance of endothelin ETA and ETB receptors in selected arterial and venous smooth muscle preparations was characterized. Endothelin-1 induced force in the saphenous and jugular veins is normally mediated by endothelin ETB-like receptors. However, desensitization or pharmacological block of these receptors reveals an endothelin ETA receptor population that is of sufficient size to mediate full endothelin-1-evoked force. Block of either endothelin ETA or endothelin ETB receptors alone is insufficient to antagonize endothelin-1-evoked force in saphenous vein. Endothelin-1-induced force in hamster aorta may also be mediated by activation of both endothelin ETA and ETB receptors. However, activation of endothelin ETB-like receptors alone is insufficient to generate a full endothelin-1 response. Sarafotoxin S6c treatment, to desensitize endothelin ETB receptors, failed to affect the responses of rat aorta and rabbit carotid artery to endothelin-1 or endothelin ETA receptor antagonists. These findings indicate that selective endothelin receptor antagonists will vary enormously in their efficacy against endothelin-induced force in different vascular beds.

Direct vasoconstrictor effects of sandimmune (cyclosporine A) are mediated by its vehicle cremophor EL: inhibition by the thromboxane A2/prostaglandin endoperoxide receptor antagonist ifetroban.

The use of cyclosporine A (CsA), a cyclic polypeptidic immunosuppressive agent, is associated with a number of cardiovascular problems. This study assessed the effects of CsA and its vehicle, cremophor EL (cremophor), on force development in isolated vascular tissue. CsA evoked a concentration-dependent increase in force (EC50 = 2.5 +/- 0.8 micrograms/ml) in the rabbit jugular vein. Cremophor alone also produced a concentration-dependent increase in force (EC50 = 39.5 +/- 10.9 micrograms/ml) that matched the CsA/cremophor response at equivalent cremophor concentrations. The cremophor-induced vasoconstriction was inhibited by the structurally distinct thromboxane A2 receptor antagonists ifetroban and glyburide, but not by indomethacin (10 microM). Ricinoleic acid also produced vasoconstriction (EC50 = 0.24 +/- 0.04 microgram/ml) that was sensitive to inhibition by ifetroban but not by indomethacin. CsA dissolved directly in ethanol produced a small increase in force that was indistinguishable from that evoked by ethanol alone. Cremophor (EC50 = 1.5 +/- 0.5 mg/ml) and ricinoleic acid (EC50 = 4.7 +/- 0.7 microgram/ml) also evoked force development in the rabbit aorta, responses that were antagonized by ifetroban. Thus, force development evoked by CsA in the rabbit jugular vein appears to be mediated primarily by its vehicle, cremophor. It is hypothesized that cremophor, by virtue of its ricinoleic acid component, evoked force development by acting as a weak thromboxane A2 agonist.

8-(Diethylamino)-octyl-3,4,5-trimethoxybenzoate inhibits cromakalim-induced 86Rb efflux from the rat aorta.

The effects of 8-(diethylamino)-octyl-3,4,5-trimethoxybenzoate (TMB-8) on cromakalim-induced 86Rb efflux and vasorelaxation in the rat aorta have been assessed. TMB-8 inhibited cromakalim (10 microM)-induced 86Rb efflux with an IC50 of 0.50 +/- 0.15 microM (n = 4; IC50 for glyburide 0.17 +/- 0.05 microM, n = 4), but it produced minimal antagonism of the cromakalim-induced vasorelaxation (rings precontracted with 30 mM KCl-PSS) at TMB-8 concentrations < or = 3 microM. TMB-8 at 10 microM produced significant inhibition of the cromakalim-induced vasodilation. Inhibition of cromakalim-induced 86Rb efflux produced by TMB-8 was little affected by raising the KCl concentration to 30 mM (inhibition by 0.5 microM TMB-8: 62.9 +/- 6.9 and 52.5 +/- 10.9% in 4.6 and 30 mM KCl, respectively; n = 4 for both). Similarly, the inhibitory effects of glyburide on the cromakalim-induced 86Rb efflux were minimally affected by this maneuver. TMB-8 was approximately equipotent against the increase in 86Rb efflux generated by either 1 or 10 microM cromakalim (IC50: 0.73 +/- 0.11 and 0.65 +/- 0.33 microM, respectively; n = 4 of both). In contrast, the glyburide IC50 was reduced approximately 10-fold by reducing the concentration of cromakalim used from 10 to 1 microM (IC50: 182.5 +/- 4.8 and 19.5 +/- 2.6 nM, respectively; n = 4 for both). We hypothesize that TMB-8 inhibits the potassium channel that is opened by cromakalim.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin receptor subtype(s) in rabbit jugular vein smooth muscle.

The goal of our study was to characterize pharmacologically the receptor subtype(s) that mediate endothelin-induced force development in the rabbit jugular vein. Endothelin-1 (ET-1), sarafotoxin S6c, and the linear endothelin peptide Ala11,15-ET-1[8-21] evoked approximately monophasic concentration-dependent increases in force development in the rabbit jugular vein (rank order of potency: sarafotoxin S6c > ET-1 > Ala11,15-ET-1[8-21]). Maximally effective concentrations of the relatively ETB-selective (in comparison to ETA) ligands sarafotoxin S6c and Ala11,15-ET-1[8-21] produced significantly less force than a maximally effective ET-1 concentration (79 and 78% of ET-1 max., respectively; p < 0.001 for both). ET-3 produced a relatively shallow concentration-force relationship. Force evoked by ET-1 was minimally affected by the relatively ETA-selective (in comparison with ETB) receptor antagonists BQ-123 and FR139317. These data indicate that the dominant functional ET receptor in rabbit jugular vein smooth muscle is of a non-ETA subtype.

Electromechanical effects of the putative potassium channel activator celikalim (WAY-120,491) on feline atrial and ventricular muscle.

Celikalim (WAY-120,491) is a putative potassium channel activator that has been shown to lower blood pressure in animal models and humans. In the present study, we have examined the effects of celikalim on contractility and ionic currents in feline cardiac muscle. Celikalim was found to decrease contractility in electrically stimulated (2 Hz frequency) left atrial and right ventricular papillary muscle preparations with IC50 values of 0.95 +/- 0.12 microM (n = 6) and 0.29 +/- 0.07 microM (n = 5), respectively. Glyburide (1 microM) reversed the celikalim-induced negative inotropy (left atrial halves). Celikalim was also shown to activate a glyburide-sensitive current in voltage-clamped isolated ventricular myocytes that reversed close to the calculated value of the potassium equilibrium potential (n = 4 cells). In addition, celikalim was found to inhibit voltage-activated calcium current (L-type) in isolated ventricular myocytes (51 +/- 2% inhibition at 1 microM; n = 4 cells). We conclude that celikalim is a potassium channel activator and hypothesize that both the negative inotropy and the glyburide-sensitive current evoked by this drug are mediated by ATP-regulated potassium channels. Inhibition of voltage-activated calcium channels by celikalim may also contribute to the negative inotropy induced by this drug.

Modulation of the delayed rectifier, IK, by cadmium in cat ventricular myocytes.

The effects of cadmium on the delayed outward potassium current (IK) were investigated in isolated cat ventricular myocytes using the single suction pipette voltage-clamp technique. IK activation was examined using peak tail currents elicited after 750-ms voltage-clamp steps to selected membrane potentials from a holding potential of -40 mV. In the presence of Cd2+ (0.2 mM), peak tail currents increased from a control value of 85 +/- 12 to 125 +/- 18 pA (n = 4). Activation curves constructed from the average peak tail-current measurements in all experiments showed that Cd2+ shifted the voltage dependence of activation to more positive potentials by 16.4 +/- 2.0 mV and increased the slope factor of the activation curve from 6.1 +/- 0.2 to 6.9 +/- 0.2 mV. In the absence of Cd2+, increases in holding potential from -30 to -70 mV had no effect on the magnitude of the peak tail currents, suggesting that the Cd(2+)-induced increase was not the result of a voltage-dependent increase in the number of available K+ channels at the holding potential. Slow voltage ramps from -70 to +70 mV revealed that Cd2+ increased the outward current at membrane potentials positive to +20 mV and shifted the voltage range in which IK inwardly rectified to more positive potentials. The fully activated current-voltage relationship was also shifted to more positive potentials by Cd2+. Cd2+ did not alter channel selectivity for K+.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of the putative potassium channel activator WAY-120,491 on 86Rb efflux from the rabbit aorta.

WAY-120,491 [(-)-(3S-trans)-2-[3,4-dihydro-3-hydroxy-2,2-dimethyl-6-(trifluoromet hox y)- 2H-1-benzopyran-4-yl]-2,3-dihydro-1H-isoindol-1-one] is a novel antihypertensive agent. We have investigated the effects of this compound on contractile force and 86Rb efflux, using the rabbit aorta, in order to assess its K channel activator properties. K channel blockers and ionic conditions thought to modulate specific K channel types have been used to provide insight into the K channel(s) affected by this compound. WAY-120,491 evoked relaxation of precontracted rabbit aortic rings and increased the rate of 86Rb efflux from strips of rabbit aorta; both effects occurring in a concentration-dependent manner. The WAY-120,491 (1 microM)-induced 86Rb efflux was inhibited by tetraethylammonium (IC50 = 0.38 mM), indicating that the increased efflux was mediated by K channels. Glyburide completely blocked the WAY-120,491 (1 microM)-evoked 86Rb efflux with 50% block occurring at a concentration of 0.48 microM. Glyburide also antagonized the WAY-120,491-induced relaxation of aortic rings. Omission of Ca from the solution bathing the aorta did not inhibit the WAY-120,491 induced 86Rb efflux but rather caused an augmentation of the response. It is concluded that WAY-120,491 may be classified as a K channel opener. Furthermore, the K channel upon which WAY-120,491 acts exhibits some characteristics normally associated with the ATP regulated K channel although the involvement of other K channel types has not been ruled out.

A Ca2+-activated K+ channel from rabbit aorta: modulation by cromakalim.

A large conductance Ca2+-activated K+ channel from rabbit aorta was incorporated into planar lipid bilayers. This channel had a conductance of 337 +/- 7 pS in symmetrical 250 mM KCl solutions and had a Na+/K+ permeability ratio of less than 0.04. In asymmetrical solutions containing 300 mM KCl cis (intracellular), 100 mM KCl trans (extracellular) or 100 mM KCl cis 500 mM KCl trans, the reversal potentials for the channel were -30 and +46 mV, respectively. This channel possessed voltage-dependent activation and cis (intracellular) Ca2+ sensitivity. Cromakalim (50 nM) added to the trans side of the bilayer significantly increased the Popen by 56% from 0.09 +/- 0.01 to 0.14 +/- 0.01 (P less than 0.01) at -40 mV without altering the single channel conductance. This effect was dose-dependent, increasing at higher cromakalim concentrations. The primary effect of cromakalim was to decrease the tau slow of the channel closed state from 266 +/- 32 to 147 +/- 17 ms which is sufficient to account for the increase in Popen of the channel in the presence of cromakalim.

The intracellular calcium stores in the rabbit trachealis.

We investigated the role of the intracellular Ca2+ stores in the regulation of the rabbit tracheal smooth muscle contraction. Carbachol (10 microM)- and 80K-induced contractions were reduced by preincubating tissues in Ca2+-free (EGTA-PSS) solution. Contractile amplitude plotted as a function of the duration of EGTA-PSS preexposure was described by a biexponential for carbachol and a monoexponential for 80K. In EGTA-PSS, a prior caffeine (50 mM)-induced contraction prevented any subsequent phasic carbachol response; the converse was also true. In contrast, prior exposure to 80K increased the amplitude of a subsequent carbachol or caffeine contraction measured in EGTA-PSS. Repletion of Ca2+ plus either 80K or a low concentration of carbachol (0.3 microM) resulted in delayed tension development. Preincubation in forskolin (10(-5) M) in PSS also delayed tension development. We propose that the internal stores, most likely the sarcoplasmic reticulum in the airway muscle function both to supply and remove Ca2+ from the cytoplasm.

Effects of hypoxia on calcium fluxes and force development in the neonatal rat atrium.

The effects of hypoxia on 45calcium fluxes and force development were studied in the resting and stimulated (high potassium/low sodium solutions or Bay K 8644) neonatal rat atrium. Under normoxic conditions, high potassium (100 mmol.litre-1)/low sodium Tyrode solution and Bay K 8644 (2.5 X 10(-5) mol.litre-1) significantly increased calcium uptake above that measured in normal Tyrode solution. High potassium/low sodium Tyrode solution elicited a sustained tonic contracture. Bay K 8644 did not increase resting tension but induced spontaneous phasic contractions in some preparations. Hypoxia failed significantly to alter resting calcium uptake but partially inhibited (50-90%) the high potassium/low sodium and Bay K 8644 induced calcium uptake (that is, the calcium uptake above that measured in normal Tyrode solution). The magnitude of the high potassium/low sodium induced contracture was increased by hypoxia (15 min). Bay K 8644 had no effect on resting tension (in five out of six experiments) after 15 min of hypoxia. Acidosis failed to affect resting (with the exception of the 210 min time point), high potassium/low sodium induced, and Bay K 8644 induced calcium uptakes and had little effect on the high potassium/low sodium induced contracture. It is hypothesised that hypoxia reduces the cells' ability to regulate calcium. Furthermore, it appears that hypoxia's effects on calcium fluxes and the high potassium induced contracture involve other mechanisms besides the associated acidosis.

Safety of lasalocid in turkeys and its compatibility with tiamulin.

An investigation involving 640 turkeys demonstrated that the inclusion of lasalocid continuously from day-old to 16 weeks of age, at levels up to 375 ppm in the feed, produced no adverse effects; furthermore, the inclusion of 125 ppm lasalocid in the feed was compatible with the administration of 250 ppm tiamulin in the drinking water continuously for five days to turkeys over the same age range.