We studied the alterations in myocardial ,-adrenergic receptoradenylate cyclase activity and muscarinic receptor density in a canine model of left ventricular (LV) failure. LV failure was characterized by a doubling of LV weight/body weight ratio (3.3±0.1 to 6.9±0.4 g/kg) and an elevation of LV end-diastolic pressure, 32±4.5 mmHg, compared with 7.7±0.6 mmHg in normal dogs. Despite a 44% increase in receptor density as measured by antagonist binding studies with 13Hldihydroalprenolol, there was a twofold decrease in receptor affinity, i.e., an increase in the dissociation constant (Kd) (5.6±0.7 to 12±1.6 nM) in heart failure. Agonist displacement of 13Hjdihydroalprenolol binding with isoproterenol in the presence and absence of 5'-guanylylimidodiphosphate [Gpp(NH)pl demonstrated a striking loss of high affinity binding sites in heart failure (51±16 to 11±5%). fl-Adrenergic receptor-mediated stimulation of adenylate cyclase and maximal stimulation with Gpp(NH)p or sodium fluoride was reduced in heart failure. There was a concomitant marked, P < 0.01, reduction in muscarinic receptor density (242±19 vs. 111±20 fmol/mg). Thus, while muscarinic receptor density fell, ,8-adrenergic receptor density actually increased in LV failure. However, a larger portion of the ,8-adrenergic receptors are not functionally coupled to the GTP-stimulatory protein (N.), as evidenced by a decrease in the fraction of receptors that bind agonist with high affinity.

To determine the importance of the direct and the indirect pressor and vasoconstrictor actions of angiotensin II (ANG II), experiments were conducted in conscious dogs 2 to 8 weeks after instrumentation with aortic catheters and aortic electromagnetic flow probes to measure arterial pressure and cardiac output. Total peripheral resistance was calculated by an on-line digital computer. Pretreatment with propranolol eliminated complicating inotropic effects of norepinephrine, released by the indirect actions of ANG II. The pressor and vasoconstrictor responses after ganglionic blockade, in either the presence or absence of arterial baroreceptor nerves, were considered to be the direct effects of ANG II. In conscious dogs, systemically administered ANG II (32 ng/kg bolus) increased mean arterial pressure by 38 +/- 3 mm Hg, total peripheral resistance by 37 +/- 2 mm Hg/L/minute, and decreased heart rate by 15 +/- 2 beats/minute. After arterial baroreceptor denervation, administration of ANG II increased mean arterial pressure by 88 +/- 7 mm Hg, total peripheral resistance by 54 +/- 4 mm Hg/L/minute, and heart rate by 12 +/- 2 beats/minute. After arterial baroreceptor denervation and ganglionic blockade with hexamethonium, administration of ANG II increased mean arterial pressure by 53 +/- 8 mm Hg, total peripheral resistance by 27 +/- 3 mm Hg/L/minute, and left heart rate unchanged. These results indicate that in the conscious dog without baroreflex buffering nearly one-half of the pressor and vasoconstrictor actions of angiotensin are not direct, but are mediated by the autonomic nervous system.

Prior physiological studies have suggested that parasympathetic control is altered in heart failure. The goal of our studies was to investigate the influence of heart failure on the muscarinic receptor, and its coupling to adenylate cyclase. Ligand binding studies using [3Hjquinuclidinyl benzilate and enriched left ventricular (LV) sarcolemma, demonstrated that muscarinic receptor density in heart failure declined 36% from a control of 5.6±0.6 pmol/mg, with no change in antagonist affinity. However, agonist competition studies with both carbachol and oxotremorine showed that it was a loss of high affinity agonist binding sites in the sarcolemma from failing LV that accounted for this difference.The functional efficacy of the muscarinic receptor was also examined. When 1 gM methacholine was added to 0.1 mM GTP and 0.1 mM isoproterenol, adenylate cyclase stimulated activity was inhibited by 15% in normal LV but only 5% in LV sarcolemma from animals with heart failure even when the reduced adenylate cyclase in these heart failure animals was taken into account. Even at 100-fold greater concentrations of methacholine, significantly less inhibition of adenylate cyclase activity was observed in LV failure as compared with normal LV sarcolemma. Levels of the GTP-inhibitory protein known to couple the muscarinic receptor to adenylate cyclase, as measured with pertussis toxin labeling, were not depressed in LV failure. Thus, the inhibitory pathway regulating LV adenylate cyclase activity is defective in heart failure. The decrease in muscarinic receptor density, and in particular the specific loss of the high affinity agonist binding component of this receptor population, appears to be the major factor underlying this abnormality.

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