Lack of interaction of angiotensin converting enzyme inhibitors with muscarinic and neuropeptide Y receptors

In studies conducted in patients undergoing cardiac catheterizations, some hemodynamic changes were observed after the acute sublingual administration of the angiotensin converting enzyme inhibitors (ACEI) captopril, enalapril, and lisinopril. These changes consisted of an increase in pulmonary artery pressure, pulmonary vascular resistance (PVR) and induction of hypoxia. The pressure changes were transitory and disappeared after 25 min. The possible mechanisms involved in these changes may relate to interactions of the ACEI with peripheral receptor systems for hormones and neurotransmitters. We have thus undertaken the task of evaluating the potential effect of ACEI on biological receptor molecules. We have begun with studies on muscarinic receptors, and the recently characterized neuropeptide Y (NPY) receptors of endothelial cells. Equilibrium binding assays with 3H-QNB have been conducted for muscarinic receptors using rat brain synaptosomes, due to its expression of multiple muscarinic receptors subtypes. In addition 125BH-NPY binding assays were conducted on intact adrenal medullary endothelial cells. Enalapril and captopril, 10(-7) to 10(-3) M, were not able to produce significant inhibition of either muscarinic or NPY receptor probes. The paradoxical changes elicited by sublingual ACEI seems not to involve interaction with muscarinic or NPY receptors

Differential antagonism by amiloride and pirenzepine of the muscarinic receptors of rat tracheal smooth muscle

Amiloride (AM) is a well known potassium sparing diuretic. The effects of AM at the cellular level include blockade of Na+/H+ exchange in several tissues and inhibition of passive sodium flux in epithelial cells. In this study we have explored the interactions of amiloride with muscarinic receptors, using isolated rat tracheal rings and compared its effects to those of the muscarinic receptor subtype-selective antagonist pirenzepine (PZ). The results obtained demonstrate the ability of AM (100 uM to 1mM) to inhibit the ACh induced rat tracheall contractions. The inhibition resulted in the reduction of the Emax values of ACh in this preparation, and the apparent Ki for AM was of 478 uM. This effect was also observed in a sodium-free choline medium, indicating that it is independent from sodium transport mechanisms sensitive to AM. In contrast to AM, PZ displayed a surmountable type of antagonism with a pA2 value of 6.52. The results demonstrate a differential antagonism by AM and PZ of the muscarinic receptors present in the smooth muscle of the rat trachea

Equilibrium kinetics modal for the cGMP-stimulated phosphodiesterase of brain coated vesicles

An equilibrium kinetics model is proposed to described some of the enzymatic properties of the cyclic GMP-stimulated phosphodiesterase activity associated with brain clathrin coated vesicles. The model assumes the presence of pharmacologically distinct regultory and catalytic domains in the enzyme. The model contemplates that random fashion occupancy of the regulatory site by the substrate, cyclic GMP, induces a conformational change which leads to the generation of a actived catalytic state. Therefore, cyclic GMP is a positive allosteric modulator of the coated vesicle enzyme. Experimental data revealed that occupancy or activation of the regulatory site was not essential for catalysis to occur since hydrolysis occured after loss (200%) of the activation by cyclic GMP. This constitutes an example of non-essential substrate activation. Analysis of this PDE following activation by cGMP and after loss of the regulation, activation capacity of the enzyme allows the calculation of the various kinetic parameters inherent in the model