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1. alpha 1-Adrenoceptor densities were studied in cardiac membrane preparations from several mammalian species including human failing hearts under identical experiment conditions; the alpha 1-adrenoceptor antagonist, [3H]-prazosin, was used as radioligand. End-stage heart failure (NYHA IV) in human hearts was due to idiopathic dilated cardiomyopathy. 2. The ventricular alpha 1-adrenoceptor densities were not significantly different in guinea-pig, mouse, pig, calf, and man (11 to 18 fmol mg-1 protein) but about 5 to 8 fold smaller than in rat (about 90 fmol mg-1 protein). Right and left ventricular receptor densities were similar in these species. 3. A sufficient amount of right and left atrial tissue was obtained from rabbit, pig, calf, and man only. The alpha 1-adrenoceptor densities in both atria of these species were found to be at the detection limit of the method used (less than 8 fmol mg-1 protein). 4. The equilibrium dissociation constant (KD) was similar in all species studied ranging from 0.047 +/- 0.006 to 0.063 +/- 0.007 nmol l-1. 5. It is concluded that differences in alpha 1-adrenoceptor density between atria and ventricles may exist in mammalian species. The exceptionally high density of these receptors in rat ventricles seem to be a particular feature in these animals.

1. alpha 1-Adrenoceptor (phenylephrine in the presence of propranolol) and beta 2-adrenoceptor (fenoterol)-mediated positive inotropic effects were investigated in human ventricular preparations isolated from five non-failing (prospective organ donors) and from eight explanted failing hearts with end-stage idiopathic dilative cardiomyopathy (NYHA IV). 2. For comparison, the nonselective beta-adrenoceptor agonist isoprenaline, the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), the cardiac glycoside dihydroouabain, and calcium were studied. 3. Furthermore, the influence of IBMX on adenosine 3':5'-cyclic monophosphate (cyclic AMP) PDE activity as well as total beta-adrenoceptor density, beta 1- and beta 2-adrenoceptor subtype distribution, and alpha 1-adrenoceptor density were compared in nonfailing and failing human heart preparations. The radioligands (-)-[125I]-iodocyanopindolol for beta-adrenoceptor binding and [3H]-prazosin for alpha 1-adrenoceptor binding were used. 4. The inotropic responses to calcium and dihydroouabain in failing human hearts were unchanged, whereas the maximal alpha 1- and beta 2-adrenoceptor-mediated positive inotropic effects were greatly reduced. The inotropic effects of the other cyclic AMP increasing compounds, i.e. isoprenaline and IBMX, were also reduced to about 60% of the effects observed in nonfailing controls. The potency of these compounds was decreased by factors 4-10. 5. The basal PDE activity and the PDE inhibition by IBMX were similar in nonfailing and failing preparations. 6. The total beta-adrenoceptor density in nonfailing hearts was about 70 fmol mg-1 protein. In failing hearts the total number of beta-adrenoceptors was markedly reduced by about 60%. The betal/beta2-adrenoceptor ratio was shifted from about 80/20% in nonfailing to approximately 60/40% in failing hearts which was due to a selective reduction of beta1-adrenoceptors. The beta2-adrenoceptor population remaining unchanged. alpha-Adrenoceptor density was increased from about 4 fmol mg-' protein in nonfailing to 10 fmol mgprotein in failing hearts.7. Changes in PDE activity and adrenoceptor downregulation cannot completely explain the reduced positive inotropic effects of alpha 1- and beta 2-adrenoceptor agonists in failing human hearts. This supports the hypothesis that impairment of other processes such as the coupling between receptor and effector system, i.e. the respective G-proteins, are equally important in end-stage heart failure.

1 Adenosine and the adenosine receptor agonist (-)-N6-phenylisopropyladenosine (PIA) produced a small positive and negative inotropic effect, respectively, in isolated electrically driven papillary muscles of guinea-pigs. 2 Adenosine (100 mumol l-1) had no effect on cyclic AMP or cyclic GMP content. PIA (100 mumol l-1) slightly increased cyclic AMP. 3 In the presence of 3-isobutyl-1-methylxanthine (IBMX; 60 mumol l-1), which increased force of contraction 2 fold, adenosine and PIA exerted strong negative inotropic effects. PIA was more potent than adenosine (mean IC25 2.1 and 168 mumol -1, respectively). 4 In contrast, the nucleosides did not affect the increase in force of contraction produced by elevating extracellular Ca2+ concentration. 5 The IBMX-antagonistic effects of adenosine and PIA were not accompanied by modification of the IBMX-induced increase in cyclic AMP and cyclic GMP. 6 The effects of adenosine and PIA on force of contraction were accompanied by a partial reversal of the IBMX-induced increase in the maximal rate of depolarization of slow action potentials. 7 It is concluded that adenosine and PIA are able to attenuate the positive inotropic effect of a phosphodiesterase inhibitor. This effect is unlikely to be due to a reduction of the IBMX-induced increase in cyclic AMP content. It is conceivably due to an inhibition of the stimulant action of cyclic AMP on slow Ca2+ channels leading to the reduction of the slow inward current which in turn reduces force of contraction.

1. It has been suggested that impaired beta-adrenoceptor stimulation is a condition under which the functional role of cardiac alpha 1-adrenoceptors is enhanced. We therefore investigated the extent and time course of changes in alpha 1-adrenoceptor characteristics after chronic treatment with the beta-adrenoceptor blocker propranolol in rat heart. For comparison beta-adrenoceptors were also studied. The mechanism of the changes in adrenoceptor density was investigated with cycloheximide, an inhibitor of protein synthesis. The functional significance of an increased alpha 1-adrenoceptor density was tested by measuring isometric force of contraction in the presence of phenylephrine or isoprenaline in right ventricular papillary muscles. 2. Rats were treated with propranolol (9.9 mg kg-1 daily) or 0.9% NaCl, applied with osmotic minipumps for 1, 2, 3 or 7 days. Propranolol treatment resulted in a maximally 28% increase of alpha 1-adrenoceptor density after 3 days (NaCl 95.9 +/- 3.5 vs. propranolol 123.0 +/- 1.6 fmol mg-1 protein, n = 6, P less than 0.01). This up regulation reached significant levels after 2 days of treatment and was reversible after cessation of treatment within two days. KD-values were the same for NaCl- and propranolol-treated rats. Changes of Bmax and KD in beta-adrenoceptor binding assays did not reach significant levels. 3. Cycloheximide (1.5 mg kg-1 i.p. daily for 3 days) inhibited the propranolol-induced increase in Bmax of alpha 1-adrenoceptors completely. In addition, cycloheximide also decreased the density of alpha 1- and beta-adrenoceptors also under control conditions. 4. pD2-values for the positive inotropic effect of phenylephrine and isoprenaline in isolated electrically driven papillary muscle were similar in NaCl- and propranolol-treated rats (phenylephrine: 5.41 + 0.11 vs. 5.41 + 0.19, n = 7; isoprenaline: 6.31 + 0.18 vs. 6.65 + 0.19, n = 7). The observed increase in alpha-adrenoceptor density in healthy rat heart may therefore not be high enough to enhance the phenylephrine-induced increase in force of contraction. 5. In conclusion, time course and effects of cycloheximide indicate that the increase in B,,,, of myocardial alpha 1-adrenoceptors is due to de novo synthesis of receptors. However, at least for the rat heart model, a functional significance of this increase could not be demonstrated.