piperidines and 5-dimethylamiloride

We examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue and myocytes from 4-week-old mice. In the presence of SEA0400, a specific inhibitor of the Na+-Ca2+ exchanger, endothelin-1 produced positive inotropy. Endothelin-1, when applied to cardiomyocytes in the presence of SEA0400, did not change the peak amplitude of the Ca2+ transient but increased intracellular pH and Ca2+ sensitivity of contractile proteins. On the other hand, in the presence of dimethylamiloride (DMA), a specific inhibitor of the Na+-H+ exchanger, endothelin-1 produced negative inotropy. In cardiomyocytes, in the presence of DMA, endothelin-1 produced a decrease in peak amplitude of the Ca2+ transient. In the presence of both DMA and SEA0400, endothelin-1 produced neither positive nor negative inotropy. Positive inotropy was blocked by BQ-123 and negative inotropy by BQ-788. These results suggested that endothelin-1-induced positive inotropy is mediated by ET(A) receptors, activation of the Na+-H+ exchanger and an increase in intracellular pH and Ca2+ sensitivity and that the negative inotropy is mediated by ET(B) receptors, activation of the Na+-Ca2+ exchanger and decrease in Ca2+ transient amplitude.

Topics: Amiloride; Analysis of Variance; Aniline Compounds; Animals; Calcium; Cardiotonic Agents; Dose-Response Relationship, Drug; Endothelin-1; Heart Ventricles; Hydrogen-Ion Concentration; In Vitro Techniques; Mice; Mice, Inbred Strains; Myocardial Contraction; Myocytes, Cardiac; Oligopeptides; Peptides, Cyclic; Phenyl Ethers; Piperidines; Receptor, Endothelin A; Receptor, Endothelin B; Sodium-Calcium Exchanger; Sodium-Hydrogen Exchangers

The effect of endothelin (ET)-1 on cardiac energetics is not fully understood.. In isolated, coronary-perfused rat hearts, we measured left ventricular contractility index (E(max)), pressure-volume area (PVA), and myocardial oxygen consumption (MVO(2)) before and after administration of ET-1 (1x10(-)(9) mol/L). ET-1 increased E(max) by 48+/-16% (P<0.01) and the total MVO(2) by 24+/-11% (P<0.01). The MVO(2)-PVA relations were linear both before and after ET-1 (r>0.99). ET-1 shifted MVO(2)-PVA upward, increasing the MVO(2) intercept by 24+/-13%. At the same time, ET-1 decreased the slope (S), with 1/S (contractile efficiency) being 46+/-5% before and 56+/-5% after ET-1 (P<0.01). ET-1-induced increases in E(max) and in contractile efficiency were abolished by an ET(A) receptor blocker (S-0139) but not by an ET(B) blocker (BQ-788). Although high [Ca(2+)] perfusion increased E(max) and the intercept to the same extent as ET-1, it did not change S. N(G)-Nitro-L-arginine (an inhibitor of nitric oxide synthase) increased the coronary perfusion pressure as much as ET-1, but S again remained unchanged. Dimethylamyloride (Na(+)/H(+) exchanger inhibitor) partially blocked the positive inotropic effect of ET-1 but not the ET-1-induced increase in the contractile efficiency.. Agonistic effects of ET-1 on the ET(A) receptor economized the chemomechanical conversion efficiency of the left ventricular unit myocardium by a mechanism independent of the Na(+)/H(+) exchanger. This unique oxygen-saving effect of ET-1 may play an adaptive role in the failing myocardium, in which local accumulation of ET-1 is present.

Topics: Amiloride; Animals; Antihypertensive Agents; Caffeic Acids; Cardiotonic Agents; Drug Interactions; Endothelin-1; Heart; In Vitro Techniques; Male; Myocardial Contraction; Myocardial Ischemia; Oleanolic Acid; Oligopeptides; Oxygen; Perfusion; Piperidines; Rats; Rats, Sprague-Dawley

Loperamide, an opiate agonist, inhibited the Na+-H+ exchanger in brush-border membrane vesicles isolated from term human placenta, rabbit renal cortex and rabbit small intestine in a dose dependent manner. Because the placental Na+-H+ exchanger was the most sensitive to inhibition by loperamide (IC50 = 60 microM), characterization of the inhibition was done with the placental Na+-H+ exchanger. The inhibition of the placental Na+-H+ exchanger by loperamide was instantaneous and freely reversible. Kinetic analyses demonstrated that the inhibition was of a mixed type. Loperamide (70 microM) reduced the maximal velocity (Vmax) from 46.4 +/- 1.2 to 34.5 +/- 1.1 nmol/mg of protein/15 s and increased the apparent affinity constant (Kt) for Na+ from 12.3 +/- 1.0 to 16.5 +/- 1.5 mM. Loperamide interacted with the exchanger protein at more than one site. The effects of loperamide were not antagonized by naloxone, suggesting the noninvolvement of opiate receptors in this process. These results differentiate loperamide from other Na+-H+ exchanger inhibitors such as amiloride, cimetidine and clonidine, which interact with the exchanger at a single site in a strictly competitive manner.

Topics: Amiloride; Carrier Proteins; Drug Interactions; Female; Humans; Kinetics; Loperamide; Microvilli; Naloxone; Piperidines; Placenta; Reference Values; Sodium-Hydrogen Exchangers