Catecholamine responsive adenylate cyclase in human myocardial preparations. Properties and optimalization of assay conditions

Effects of NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid) on chloride transport in intestinal tissues and the T84 cell line

NPPB (5-nitro-2-(3-phenylpropylamino)benzoic acid) has been reported to block Cl- channels in isolated rabbit nephrons with high potency (IC50 = 80 nM). The effects of this compound on Cl(-)-mediated transport processes in intestinal tissues have been studied using agonist-stimulated short-circuit current (T84) in Ussing chamber experiments and 36Cl- fluxes in monolayers of a colonic cell line (T84). NPPB inhibited PGE1-stimulated Isc in rabbit distal colon and ileum at concentrations in the range 20 to 100 microM. However, NPPB at the same concentrations also inhibited glucose-stimulated Isc in rabbit ileum, suggesting that its effects were not restricted to those on Cl- transport. Consistent with this, exposure of rabbit distal colon to 100 microM NPPB was found to reduce endogenous ATP levels by 69%, implying that, at these concentrations, NPPB could impair active transport processes by an effect on cellular energy metabolism. Clear evidence for a direct effect of NPPB on epithelial chloride channels was found in studies on Cl- fluxes in T84 cell monolayers. NPPB inhibited VIP-stimulated Cl- uptake into T84 cells with an IC50 of 414 microM. NPPB (1 mM) also inhibited Cl- efflux from pre-loaded cells confirming its effect as a weak Cl- channel blocker in this system.

Relative selectivity of different beta-adrenoceptor antagonists for human heart beta 1- and beta 2-receptor subtypes assayed by a radioligand binding technique

The affinity constants of inhibition (Ki values) for both beta 1- and beta 2-receptor subtypes were determined for four different beta-adrenoceptor antagonists by a radioligand binding technique in a human myocardial membrane preparation. The radioligand was the high affinity antagonist [125I]-(-)-iodocyanopindolol (ICYP), and the drugs tested were atenolol, metoprolol, ICI 141,292 and ICI 118,551. Different concentrations of the drugs at test were allowed to compete with a constant concentration of ICYP for the specific binding sites (beta-receptors). Ki values for beta 1- and beta 2-receptors for each beta-adrenoceptor antagonist were developed from these data by computer calculations. Atenolol and metoprolol were found to differ slightly regarding potency (absolute Ki values) and to be practically equal regarding relative selectivity (approx. 40; i.e. ratio between high and low Ki values), while ICI 141,292 was found to have slightly higher relative selectivity (approx. 60) and much higher potency. All these drugs exhibited highest affinity for the beta 1-receptor population. In contrast, ICI 118,551 exhibited a very high relative selectivity (approx. 300) with highest affinity for the beta 2-receptor subtype. The method represents a good supplement to physiological and clinical examinations of selectivity of beta-blockers, and offers several advantages regarding simplicity, specificity and accuracy.

Relative potencies of various beta-adrenoceptor antagonists (BAA) at the level of the human myocardial beta-adrenoceptor-adenylate cyclase (AC) complex. Is intrinsic sympathomimetic activity (ISA) due to AC activation?

Nine different beta-adrenoceptor antagonists (BAA), five with intrinsic sympathomimetic activity (ISA), were examined for their ability to inhibit isoproterenol-stimulated adenylate cyclase (AC) activity and specific 125I-cyanopindolol (CYP) binding in crude membrane particles from human myocardium. The BAA's were: propranolol, pindolol, timolol, alprenolol, metoprolol, atenolol, prenalterol, ICI 141.292 'Visacor', and ICI 118.587 'Corwin'. Whether BAAs with strong ISA were able to stimulate AC activity by themselves were examined in separate experiments and compared to the AC stimulation by full agonists. All the BAAs caused a concentration dependent, and at high doses apparently complete, inhibition of both isoproterenol-stimulated AC activity and 125I-CYP binding. Both assays made possible a 'potency-ranking' of the different BAAs (pindolol greater than or equal to propranolol and timolol greater than ICI 142.292 and alprenolol greater than ICI 118.587, prenalterol and metoprolol greater than atenolol). Corrected IC50-values, derived from inhibition curves with both techniques, show that receptor binding and inhibition of receptor function follow each other closely. Prenalterol caused a very weak AC activation (5.4% of maximum), whereas the 'ISA-blockers', pindolol, ICI 141.292, and ICI 118.587 were unable to stimulate AC activity at concentrations which completely displaced 125I-CYP binding. In comparison, norepinephrine stimulated AC activity to the same level as isoproterenol (three to four times basal activity) and the beta 2-selective agonist terbutaline caused some 50% of maximal AC stimulation. This raises the question whether ISA is due to AC activation. The effect upon AC activation and 125I-CYP binding of drugs with beta-selectivity shows that both beta 1- and beta 2-receptors are coupled to the AC.

Catecholamine-responsive adenylate cyclase activity in human endomyocardial biopsies. Individual sensitivity to isoproterenol stimulation and propranolol inhibition

In the present study we have examined adenylate cyclase (AC) activity, the stimulation by isoproterenol and inhibition by propranolol, in endomyocardial biopsies from eleven patients with suspected cardiomyopathy. Biopsies were obtained by heart catheterization from the right endomyocardial surface of the interventricular septum. Three biopsies were taken from each patient (mean weight, 2.1 mg; range, 1.2-4.0 mg). One biopsy was studied by light microscopy. The two other biopsies were homogenized and AC activity in the homogenates was determined in the presence of different concentrations of isoproterenol and isoproterenol (5 micrograms/ml) combined with different concentrations of propranolol. Thus stimulation and inhibition curves were established for a pair of biopsies from each patient. Appropriate biopsy material was obtained in triplicate from only seven patients. In these patients the variance in maximal receptor stimulation (by isoproterenol) and inhibition (by propranolol) was significantly smaller in pairs of biopsies compared to the variance between all biopsies (p values from less than 0.05 to less than 0.025). Hence it is possible to determine AC activity, and adrenergic receptor function, in very small endomyocardial biopsies. New diagnostic possibilities could thereby be introduced.