Role of nitric oxide in systemic hemodynamic responses to dobutamine, epinephrine, and amrinone.

OBJECTIVE: This study was designed to investigate the extent to which the systemic vasodilator effects of dobutamine, epinephrine, and amrinone are modulated by the endothelium-derived relaxing factor, nitric oxide (NO). DESIGN: This was a prospective study of low and high doses of the agonists before and after inhibition of NO synthesis. SETTING: Experiments were performed in the basic research laboratories of the Center for Anesthesiology Research. PARTICIPANTS: Pentobarbital-anesthetized, intact Sprague-Dawley rats were studied in seven separate groups of eight rats each. INTERVENTIONS: The systemic vasodilator responses to the agonists were assessed before and after the administration of the NO synthase inhibitor, NG-nitro-L-arginine methyl ester. MEASUREMENTS AND MAIN RESULTS: Decreases in systemic vascular resistance in response to dobutamine and epinephrine were not observed after inhibition of NO synthesis, whereas the decrease in systemic vascular resistance in response to amrinone was still apparent. CONCLUSIONS: The results suggest that dobutamine and epinephrine produce systemic vasodilation through the release of NO, whereas amrinone produces vasodilation independent of NO release.

Hemodynamic responses to dobutamine during acute normovolemic hemodilution.

The effects of dobutamine (DOB) on myocardial performance, systemic hemodynamics, and oxygen delivery during acute normovolemic hemodilution in anesthetized rats were studied. Forty-two Sprague Dawley rats (body weight 375 to 425 g) were divided into six equal groups. Hemodynamic and cardiac indices were measured or calculated at baseline, 30 minutes after the initiation of hemodilution (HD), and 15 minutes after DOB or saline infusion. Myocardial performance in response to acute pressure or volume loads was studied in all groups of animals. HD to a hematocrit (Hct) value of 20% resulted in no change in heart rate (HR), increased CI, SVI, and LV dP/dt, and decreased MAP, SVRI, and oxygen delivery index (O2DI). HD increased peak SV and CI after preload stress while the left ventricular developed pressure (LVDP) was unchanged. Infusion of DOB as 7.5 or 15 micrograms/kg/min increased HR, CI, and LV dP/dt as well as LVDP. At the same time, DOB decreased MAP and SVR, whereas the SVI remained unchanged. In non-HD animals both doses of DOB increased LVDP, but only the larger dose increased CI, whereas peak SV decreased with the smaller dose. Arterial carbon dioxide tension (PaCO2) increased, whereas pH and arterial oxygen tension (PaO2) decreased; however, O2DI remained unchanged. Concomitant hemodilution and DOB infusion resulted in attenuation of HR response to DOB, exaggerated the drop in MAP and SVR, and increased LV dP/dt. Only the larger dose of DOB increased the CI, whereas neither dose could alter the SVI in HD animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural and functional consequences of minoxidil-induced cardiac hypertrophy.

Minoxidil, a potent arteriolar vasodilator, is used clinically as an antihypertensive agent, providing blood pressure control in patients who are resistant to conventional drug therapy. In spite of its antihypertensive effects, however, minoxidil treatment does not result in regression of cardiac hypertrophy. Laboratory studies have shown that minoxidil treatment actually causes cardiac enlargement when administered to normotensive animals. The mechanism of minoxidil-induced cardiac hypertrophy and its consequences for the myocardium have not been determined. Because cardiac hypertrophy is a significant independent risk factor for cardiovascular morbidity and mortality, it is important to understand this dissociation between blood pressure and cardiac enlargement and also to consider the possible impact of such findings on the clinical use of minoxidil. In the investigation presented here, we examined the structure and function of cardiac muscle from normotensive rats who developed myocardial hypertrophy after minoxidil treatment. Data demonstrate that minoxidil produces enlargement of the left ventricle, right ventricle and interventricular septum. The right ventricular enlargement produced by minoxidil treatment is due to an increase in myocyte cross-sectional area, a finding which may suggest pressure overload of the right ventricle. Left ventricular and septal enlargement in this model cannot be totally accounted for by an increase in myocyte cross-sectional area, suggesting that minoxidil causes enlargement by a mechanism other than pressure overload in these areas of the heart. Functionally, left ventricular papillary muscles from hearts with minoxidil-induced hypertrophy develop force which is equal to that developed by control muscles, but contract and relax more slowly than muscles from control hearts.(ABSTRACT TRUNCATED AT 250 WORDS)

Contrasting effects of vecuronium and succinylcholine on the renal microcirculation in rodents.

We assessed the effects of succinylcholine and vecuronium on renal function and on the renal microcirculation in a rodent model. Vecuronium (0.02 mg/kg followed by 0.2 mg.kg-1 x h-1) caused a significant decrease of 16.1% +/- 3.87% in inulin clearance from 0.92 +/- 0.07 to 0.71 +/- 0.05 mL.min-1 x gKW-1 (gram of kidney weight), and a decrease in para-aminohippuric acid clearance by 21.6% +/- 4.69% from 1.58 +/- 0.26 to 1.31 +/- 0.20 mL.min-1 x gKW-1 (P < 0.05), whereas succinylcholine (0.45 mg/kg followed by 2 mg.kg-1 x h-1) altered neither. The effect of these muscle relaxants was also determined on the renal microcirculation in separate experiments using videomicroscopy. Succinylcholine (n = 10; 10(-10) to 10(-6) M) and its parent compound, acetylcholine (n = 10, 10(-10) to 10(-6) M) used as a control, caused a significant vasodilation from baseline diameter in the interlobular, afferent, and efferent arterioles. The vasodilation caused by succinylcholine was significantly less than that observed with acetylcholine. Atropine blocked the response to succinylcholine, indicating the latter has a muscarinic effect. In contrast, vecuronium caused a significant, selective vasoconstriction from baseline diameter in the preglomerular vessels, but not in the postglomerular vessels. The vasoconstriction caused by vecuronium was significantly different than the vasodilation caused by succinylcholine. The preglomerular vasoconstriction observed with vecuronium may contribute to the decrease in renal plasma flow and glomerular filtration rate observed experimentally. The choice of a neuromuscular blocking drug can therefore have the potential to influence renal function by altering the renal microcirculation.

Fentanyl attenuates porcine coronary arterial contraction through M3-muscarinic antagonism.

The "antimuscarinic effect" of fentanyl and its dependence on subtypes of receptors were characterized in isolated porcine coronary arteries. Left anterior descending coronary arteries were dissected from the hearts of 60 adult pigs obtained at a slaughterhouse and prepared for isometric tension studies. The effects of fentanyl on the cumulative concentration-response curve for acetylcholine were obtained in the presence and absence of muscarinic blockade by atropine. Fentanyl shifted the concentration-response curve to the right in a concentration-dependent fashion. Atropine shifted the concentration-response curve to the right, and no further shift was caused by fentanyl. To investigate the dependence on muscarinic receptor subtypes, the effect of fentanyl on acetylcholine-induced contraction was examined in the presence of specific M1-, M2-, and M3-muscarinic antagonists. The pA2 values for fentanyl decreased significantly in the presence of atropine (a nonspecific antagonist) and also in the presence of p-F-HHSiD (an M3-antagonist). In contrast, no significant change of pA2 value for fentanyl was observed in the presence of both pirenzepine (an M1-antagonist) and methoctramine (an M2-antagonist). We conclude that fentanyl has an antimuscarinic effect, and that this antagonism occurs in a competitive manner. Furthermore, the significant decrease of the pA2 value for fentanyl in the presence of M3-, but not in the presence of M1 + M2-antagonists, suggests that the attenuation of cholinergic contraction of porcine coronary arteries by fentanyl is mediated through the M3-muscarinic receptor subtype.

Acute morphological effects of cocaine on rat cardiomyocytes.

The cardiovascular actions of cocaine in vivo are varied and often antagonistic. Cocaine produces excitatory sympathomimetic effects by interfering with re-uptake of norepinephrine at adrenergic nerve terminals. However, the local anaesthetic properties of cocaine exert depressant effects on the myocardium. In an attempt to differentiate the direct effects of cocaine from the indirect sympathomimetic effects on the myocardium, primary cultures of neonatal rat cardiomyocytes were established under serum-free conditions and exposed to cocaine and/or norepinephrine. After 36-48 h in culture, spontaneously contracting cells were treated with cocaine (10-1,000 micrograms/ml) and contractile rate (beats/min) quantitated, after which the cells were processed for ultrastructural examination. The contractile rate was reduced at all dosages with nearly 80% reduction at the highest concentration studied. Recovery of beating rate was observed 24 h after removal of cocaine. Pronounced cytoplasmic vacuolation of the cells occurred at concentrations > or = 100 micrograms/ml. Ultrastructural examination revealed extensive myofibrillar disruption, membrane damage, and a near complete loss of organized sarcomeres. Nuclear morphology remained unaffected. Within 24 h after removal of cocaine from the medium, myocytes recovered their characteristic cytoplasmic architecture, indicative of sarcomere reassembly. The results observed in response to cocaine were distinctly different from the response to norepinephrine. In these myocytes, the contractile rate was enhanced twofold and morphological damage was not observed. These findings suggest that cocaine can directly alter myocardial morphology independent of its sympathomimetic effects.

Effects of opioids on vasoresponsiveness of porcine coronary artery.

Myocardial ischemia during surgery can be caused by coronary vasospasm. Neurohumoral mechanisms are involved in this phenomenon, and various substances have been suggested as possible causes, including acetylcholine, histamine, and norepinephrine. The responses of isolated porcine coronary arteries (from 117 pig hearts) with (E+) and without (E-) endothelium to these agents were investigated in the presence of fentanyl, sufentanil, and morphine. Fentanyl significantly shifted to the right, in a concentration-dependent fashion, the concentration-response curve to acetylcholine. This effect was not different between E+ and E- rings. Neither sufentanil nor morphine altered acetylcholine-induced contraction of porcine coronary arteries. Naloxone did not antagonize the suppressive effect of fentanyl on acetylcholine-induced contraction. The response of porcine coronary arteries to norepinephrine was decreased only at very high concentrations of fentanyl. Neither sufentanil nor morphine altered norepinephrine-induced contraction of porcine coronary arteries. Fentanyl, sufentanil, and morphine had no effect on histamine-induced contraction. We conclude that fentanyl antagonized acetylcholine-induced contraction of porcine coronary arteries. This effect of fentanyl seems to be caused by a direct effect on smooth muscle cells and is not opioid-receptor mediated.

Cyclic AMP in myocytes isolated from hypertrophied rat hearts.

Impaired inotropic responsiveness to isoproterenol stimulation has been reported in the hypertrophied hearts of spontaneously hypertensive rats and renal hypertensive rats. This study was carried out in order to investigate the possibility that a defect in cyclic AMP production by cardiac myocytes is responsible for the impaired inotropic responsiveness of these hearts. Basal and isoproterenol stimulated cyclic AMP levels were measured in ventricular myocytes isolated from hypertrophied rat hearts. Cyclic AMP accumulation was also measured in the presence of isobutyl-methyl-xanthine, a phosphodiesterase inhibitor, and the results were compared to the appropriate controls. In the spontaneously hypertensive rat, no changes were detected in the basal or isoproterenol stimulated cyclic AMP formation. This suggests that the biochemical alterations leading to a diminished inotropic response in this model of cardiac hypertrophy involve abnormalities in mechanisms other than cyclic AMP production. In the renal hypertensive rat, basal and isoproterenol stimulated cyclic AMP levels were significantly depressed as compared to controls. This suggests that abnormalities in the signal transduction mechanism and formation of cyclic AMP are, at least in part, responsible for the impaired inotropic responsiveness seen in this model. These results confirm that cardiac hypertrophy is a heterogeneous process. Reduced inotropic responsiveness to isoproterenol stimulation in the hypertrophied hearts of the SHR and the RHR, both models of pressure overload hypertrophy, involve different biochemical alterations. Results of this study suggest that the physiologic response of cardiac hypertrophy may not be as important as the underlying cause of hypertrophic stimuli in determining the pathophysiological consequences.

Circulatory effects of verapamil during normovolemic hemodilution in anesthetized rats.

Patients who have undergone perioperative normovolemic hemodilution may require calcium channel blockers for the treatment of myocardial ischemia and/or supraventricular tachyarrhythmias. The purpose of this rodent study was to examine the effect of intravenous verapamil on the hyperdynamic circulatory response to acute normovolemic hemodilution (hematocrit 20%). Anesthetized animals were randomly divided into four groups equal in number: (1) controls (no hemodilution, no drug); (2) hemodilution only; (3) verapamil only; and (4) hemodilution followed by verapamil. Cardiac output was recorded using an electromagnetic flow probe. Pre- and afterload tests were performed, the former consisting of rapid infusion of blood adjusted for hematocrit, the latter consisting of an aortic clamp technique. Animals in group 2 had significantly (P less than 0.05) greater percent increases in cardiac index, stroke volume index, and dP/dt, and greater percent decreases in mean arterial pressure, systemic vascular resistance, and oxygen delivery than did control animals (group 1). Infusion of verapamil after hemodilution (group 4) did not interfere with the compensatory increases in cardiac index and stroke volume index seen in group 2, nor did it reduce the peak stroke volume index in response to preload stress, although it did reduce resting dP/dt, mean arterial pressure and systemic vascular resistance, and peak cardiac index and "left ventricular developed pressure" after preload and afterload stress, respectively. We conclude that reduced ventricular function after verapamil administration does not interfere with the compensatory increase in stroke volume index after normovolemic hemodilution.

Effects of propranolol on myocardial performance during acute normovolemic hemodilution.

The effects of propranolol and hemodilution on myocardial performance and oxygen delivery were evaluated in 36 anesthetized rats. Oral propranolol treatment consisted of 64 mg/kg/d for 6 weeks prior to the experiments, whereas intravenous (IV) propranolol treatment consisted of 5 micrograms/kg/min for 60 minutes after hemodilution. The hematocrit was reduced to 20% by a hetastarch-for-blood exchange. Animals were divided into six equal groups as follows: (1) no oral drug (water), no hemodilution, no IV drug (saline); (2) oral water, hemodilution, IV saline; (3) oral water, no hemodilution, IV propranolol; (4) oral water, hemodilution, IV propranolol; (5) oral propranolol, no hemodilution, IV saline; and (6) oral propranolol, hemodilution, IV saline. Left ventricular (LV) pressure, maximal dP/dt, ascending aortic blood flow, and response to preload (peak cardiac and stroke volume indices) and afterload (LV-developed pressure) stress were measured. In group 2, hemodilution significantly increased cardiac index, stroke volume index, and dP/dt, and decreased blood pressure, peripheral resistance, and oxygen delivery compared with group 1. Compared with group 2, IV propranolol after hemodilution in group 4 significantly decreased cardiac index, dP/dt, LV-developed pressure, and peak cardiac index, and increased peripheral resistance. Stroke volume index and peak stroke volume index after preload stress remained elevated in group 4, despite the negative inotropic effects of IV propranolol. Oral propranolol in group 6 did not prevent the hemodilution-induced increase in stroke volume index and peak stroke volume index in response to preload stress, although it did decrease cardiac index and dP/dt compared with group 2. Oxygen delivery was reduced in the hemodiluted animals in proportion to the decrease in hemoglobin, regardless of propranolol treatment. It is concluded that reduced myocardial contractility and cardiac performance by nonselective pharmacological beta-adrenoceptor blockade does not interfere with the compensatory increase in stroke volume index after hemodilution.

Portal-like microcirculation in rat sympathetic ganglia.

The fine structure of intraganglionic blood vessels of rat superior cervical sympathetic ganglia is studied with the light microscope and with both conventional and ultrastructural histochemical methods. Two sets of small capillaries together with larger sinusoidal ones are identified. One set of capillaries is associated with the clustered (type II) small catecholamine-containing (CC) cells and exhibits features suggestive of fluid transport function (multiple wide fenestrae and active pinocytosis). The second set of capillaries is in direct relation to the sympathetic neurons (SN) and shows characteristics suggestive of absorptive function (microvilli and pinocytotic vesicles). The larger sinusoidal capillaries are observed in the vicinity of type II CC cells, extend parallel to the long axes of the perikarya of the neurons and occasionally form loops around them. The latter are assumed to be larger blood spaces connecting the two capillary sets and serve to slow the circulation around the neurons. A pattern of portal-like intraganglionic microcirculation through which type II CC cells participate in modulating the SN is postulated. Type II CC cells secrete a catecholamine modulator which, driven by concentration gradient, gains access to the circulation through the fenestrated capillaries. The sinusoidal capillaries serve to perfuse the SN with a slow stream of blood rich in the catecholamine modulator. The latter can be filtered through the microvilli and pinocytotic vesicles of the second set of capillaries to induce slow inhibitory postsynaptic potential on the SN.

Propranolol-induced changes in the catecholamine-containing cells and vesicles of SHR sympathetic ganglia.

The ultrastructural effects of long-term propranolol administration on the catecholamine-containing (CC) cells of cervical sympathetic ganglia were studied in spontaneously hypertensive rats. After 2 months of propranolol administration, there were hydrops of mitochondria, together with swelling and vesiculation of Golgi complex and rough endoplasmic reticulum. Such swollen organelles seemed to coalesce with each other, resulting in transformation of major parts of the cytoplasm into large membrane bound saccules. In that group of animals, the number of CC vesicles showed a significant decrease (P less than 0.025) compared to the control. Moreover, such vesicles looked more electron dense, with decreased electron-lucent haloes compared to the control. Blood pressure recording via rat tail cuff showed a significant drop (P less than 0.0001) of the mean systolic blood pressure in the treated animals. One month after stopping propranolol administration, resorption of the cytoplasmic saccules occurred, but the cytoplasm looked lighter and lost its granular appearance. The mitochondria regained their normal shape, though they appeared fewer in number. The CC vesicles were significantly reduced in number (P less than 0.001) and were less electron dense compared to the control. The possible implications of these findings on the mechanism of action of propranolol as an antihypertensive agent are discussed.

Cocaine-induced small vessel spasm in isolated rat hearts.

Cocaine abuse has been associated with pathologic cardiovascular events including acute myocardial infarction (AMI) and sudden death. Although coronary vasospasm has been proposed as a possible mechanism, the ability of cocaine to induce coronary spasm has not been conclusively demonstrated. In these studies, isolated rat hearts were perfused with cocaine (100 micrograms to 500 micrograms/ml) for 1 minute, perfusion-fixed with glutaraldehyde, and histologically assessed for evidence of coronary spasm through light and electron microscopy. Light micrographs revealed that cocaine induced spasm in coronary arterioles up to 65 microns in diameter, whereas larger caliber vessels did not constrict. Ultrastructurally, vacuolation was observed in the endothelial and smooth muscle cells of constricted arterioles. Endothelial integrity was maintained and interendothelial junctions remained intact. Morphologic evidence of constriction was supported by data obtained from Langendorff-heart preparations in which cocaine reduced myocardial flow rate under constant pressure conditions and increased aortic perfusion pressure under constant flow conditions. Spasm induced by cocaine was prevented by the calcium entry blocker nitrendipine, but not by phentolamine, an alpha-adrenergic antagonist. The finding of small vessel spasm in this study may explain the significant number of clinical cases of cocaine-associated AMI in which the main coronary arteries appear angiographically normal.

Insulin-like growth factors and neonatal cardiomyocyte development: ventricular gene expression and membrane receptor variations in normotensive and hypertensive rats.

Defined factors regulating or influencing mammalian ventricular myocyte (cardiomyocyte) development are not known at this time. During early neonatal ventricular growth, cardiomyocytes begin a 'transition phase' of development toward cellular maturation (hypertrophy) that entails terminal proliferation and cellular binucleation. Insulin-like growth factor-I and -II (IGFs) are believed to play a major role in mammalian postnatal and fetal growth, possibly functioning in local environments which facilitate autocrine or paracrine tissue growth characteristics. Therefore, we examined the expression of the IGF genes and their corresponding membrane receptors in ventricles of normotensive and spontaneously hypertensive (SHR) rat pups during the first 7-14 days of age. We have determined: (1) by receptor crosslinking that neonatal ventricular membranes possess type 1 and type 2 IGF receptors; (2) by receptor binding analysis that type 1 IGF receptor concentration is elevated between days 1-7 in the SHR and shows an age-related decline in concentration and an increase in affinity in both strains; (3) by Northern blot analysis that neonatal rat ventricular tissue expresses primarily IGF-II RNA transcripts of 3.6, 2.3 and 1.7 kilobases (kb) in size, with low levels of IGF-I transcripts detected; (4) by slot-blot hybridization that SHR ventricles contain higher levels of IGF-II transcripts at 3 days of age; and (5) localized the IGF transcripts to ventricular myocytes by tissue in situ hybridization. These observations support a role for cardiomyocyte-produced IGFs that may be locally produced and act in an autocrine or paracrine fashion to modulate cardiomyocyte growth and maturation in the developing rat heart. Because both IGF receptor and IGF RNA transcript parameters differed in SHR hearts, genetically predisposed to hypertrophy, a potentially important biochemical alteration may be associated with the fetal/neonatal growth abnormalities of the developing heart in this rat strain.

Measurement of immunoreactive angiotensin peptides in rat tissues: some pitfalls in angiotensin II analysis.

Angiotensin II, the major effector peptide of the renin-angiotensin system, is an endocrine and paracrine regulator of tissue function. To determine its physiological role, it is important to quantify angiotensin II and related fragment peptides in tissues and plasma as a first step toward understanding angiotensin II metabolism within tissues. A fully characterized, sensitive, and reproducible immunochemical assay has been developed for quantitating angiotensin II immunoreactivity in tissues and plasma. We identified two methodological events of critical importance, incompletely addressed in previously reported studies. First, the nonspecific interference resulting from Sep-Pak processing was found to be due to hydrophobic impurities in the octade-casilane absorbent which were eliminated by washing the Sep-Pak with tetrahydrofuran and hexane before use. Second, a significant discrepancy was observed in the recoveries of angiotensin II and 125I-angiotensin II added to tissue extracts following high-pressure liquid chromatography. Angiotensin II immunoreactivity extracted from decapitated rat adrenal gland, brain, and kidney (target organs for angiotensin II), ovary and uterus (potential target organs for angiotensin II), and plasma has been characterized. The predominant component of the angiotensin II immunoreactivity was the biologically active octapeptide angiotensin II. However, in the brain, the ratio of angiotensin II to C-terminal angiotensin II immunoreactive fragments was lower than observed in other tissues studied. Other angiotensin II C-terminal immunoreactive peptide fragments-the biologically active heptapeptide and the biologically inactive angiotensin(3-8) and angiotensin(4-8)--were also detected in variable quantities in the various tissues.

New synapses associated with the granule-containing cells of rat sympathetic ganglia.

The synapses of the rat superior cervical sympathetic ganglion were studied with both conventional and ultrastructural histochemical methods. Besides the cholinergic synapses polarized from preganglionic fibers to sympathetic ganglion neurons, two morphologically and functionally different types of synapses were observed in relation to the small granule-containing (catecholamine-containing) cells of the rat superior cervical ganglion. The first type is an efferent adrenergic synapse polarized from granule-containing cells to the dendrites of the sympathetic ganglion neurons. This type of synapse might mediate the inhibitory effects (slow inhibitory postsynaptic potentials) induced by catecholamines on the sympathetic neurons. The second type is a reciprocal type of synapse between the granule-containing cells and the cholinergic preganglionic fibers. Through such synapses, these cells could exert a modulating effect on the excitatory preganglionic fibers. Therefore, we propose that these cells, through their multiple synaptic connections, exhibit a local modulatory feedback system in the rat sympathetic ganglia and may serve as interneurons between the preganglionic and postganglionic sympathetic neurons.

Alterations of myocardial alpha 1-adrenergic receptors in hypertensive cardiac hypertrophy in the rat.

The hypertrophied myocardium of 2 kidney-1 clip renal hypertensive rats (2K-1C RHR) displays reduced responsiveness to phenylephrine stimulation. We have studied the effects of hypertrophy on alpha 1-receptor binding properties in three models of hypertension. Specific binding density of an alpha 1-selective antagonist [3H]-prazosin to ventricular membrane of 6 and 10 weeks 2K-1C RHR was significantly decreased (45 +/- 9 fmol/mg protein, n = 14, vs 64 +/- 7.8 fmol/mg protein, n = 20, p less than .001) compared to age matched sham operated control group, but with no change in the total alpha 1-receptor content in the hypertrophied ventricles. The 4 week 1K-1C RHR showed a 29% decrease in alpha 1-adrenergic receptor density with no change in affinity. In contrast, the spontaneously hypertensive rats (SHR) showed an increase in alpha 1-receptors in the prehypertensive stage (4 weeks old) (115.2 +/- 4.9 fmol/mg protein, n = 10, vs 88.7 +/- 12 fmol/mg protein, n = 10, p less than .001) and in the adult stage with established hypertension (15 weeks old) (87.1 +/- 10.5 fmol/mg protein, n = 10, vs 67.3 +/- 9 fmol/mg protein, n = 10, p less than .001) compared to age matched or heart weight matched Wistar Kyoto rats (WKY). Reduced responsiveness to phenylephrine in RHR may be due to reduced density of alpha 1-receptors, but the reason for the increase in alpha 1-receptors in SHR's myocardium remains unclear.

Adenylate cyclase activity during development and reversal of cardiac hypertrophy.

Inotropic responses to isoproterenol of hypertrophied hearts have been shown to be decreased. We have previously reported that in 13-week-old spontaneously hypertensive rats (SHR) this decrease is probably due to decreased beta-adrenergic receptor number, while in hearts from two kidney-one clip renal hypertensive rats (2K-1C RHR), this is due to a decreased nucleotide regulatory protein activity. We now show that changes in 2K-1C RHR are time dependent. One week after instituting development of hypertension the heart is already hypertrophied. Biochemical changes consistent with decreased glucagon receptors are seen, as well as beginning changes consistent with decreases in the nucleotide regulatory protein activity. By two weeks this is more evident. Hypertrophy and biochemical changes can be reversed up to six weeks, but by ten weeks the activity of the catalytic subunit of the adenylate cyclase system is decreased. In 1K-1C RHR, biochemical changes in the cyclase system are accelerated as compared with the 2K-1C model. In SHR, changes in 24-week-old rats are the same as in the 13-week-old rats. It is concluded that in cardiac hypertrophy associated with different models of hypertension the decreased inotropic responsiveness to isoproterenol is associated with different biochemical defects in the beta-adrenergic receptor response coupling pathway, and that reversal in function occurs only when there is no apparent change in the catalytic subunit of the adenylate cyclase complex.

Impaired inotropic responses to alpha-adrenergic stimulation in experimental left ventricular hypertrophy.

We have previously reported that left ventricular hypertrophy in two-kidney, one-clip renal hypertensive rats (2K-1C RHRs) was associated with diminished inotropic responsiveness to isoproterenol and glucagon, suggesting an alteration in the receptor-adenylate cyclase cascade. The present study was performed to investigate the hypothesis that in these same hearts, inotropic responses to alpha-adrenergic stimuli could be enhanced as a compensatory mechanism. alpha-Adrenergic stimulation was achieved by graded phenylephrine infusion (1.02 to 41.2 microM/min) in the presence of propranolol (10(-7) M). The inotropic response was evaluated in the isovolumetric isolated rat heart (Langendorff preparation) paced at 260 beats/min. Results showed a significantly reduced inotropic response to alpha 1-adrenergic stimulation in 2K-1C RHR hearts irrespective of perfusion pressure (50 or 80 mm Hg [PP50 or PP80]) (+427.5 +/- 62.1 vs +1236 +/- 216.4 mm Hg X sec-1 at PP50, p less than .01 and +339 +/- 98.3 vs +1440 +/- 254 mg Hg X sec-1 at PP80, p less than .001) even when comparison was made at equivalent myocardial flow rates (RHR hearts perfused at 80 mm Hg vs control hearts perfused at 50 mm Hg). Quantitative assessment of number of alpha 1-adrenergic receptors (3H-prazosin binding) showed a significant decrease compared with that in age-matched sham-operated normotensive control rats (45 +/- 2.5 vs 64 +/- 1.7 fmol/mg protein, p less than .001).(ABSTRACT TRUNCATED AT 250 WORDS)

Beta-adrenergic receptors in rat myocardium during the development and reversal of hypertrophy and following chronic infusions of angiotensin II and epinephrine.

The effect of cardiac hypertrophy on beta-adrenergic receptor density and affinity was studied under 4 experimental conditions: in the spontaneously hypertensive rat (SHR), in the 2K-1C renal hypertensive rat (RHR), and following subcutaneous infusions of 2 pressor agents; epinephrine (E) and angiotensin II(AII). Using the antagonist 3H-dihydroalprenolol [( 3H]-DHA), the number of binding sites was shown to significantly decrease at both 13 and 24 weeks of age in the SHR when compared to age-matched WKY, with no change in affinity. In the RHR a significant increase in binding sites was observed at both 6 and 10 weeks after clipping, returning towards normal levels following removal of the clipped kidney. Cardiac hypertrophy and hypertension were induced by subcutaneous infusions for up to 2 weeks of both E and AII. E caused an alteration in receptor density, causing a significant decrease with no change in affinity. In contradistinction, although the degree of hypertrophy was the same following AII, no changes in receptor density or affinity were seen. These present experiments confirm our hypothesis that different models of hypertensive hypertrophy are associated with varying changes in beta-adrenergic receptors. This suggests that any consequential changes in myocardial function may be a result of other post receptor mechanisms.