Influence of a Na⁺-H⁺ exchange inhibitor ethylisopropylamiloride, a Na⁺-Ca²⁺ exchange inhibitor KB-R7943 and their combination on the increases in contractility and Ca²⁺ transient induced by angiotensin II in isolated adult rabbit ventricular myocytes

Abstract.

In rabbit, ventricular myocytes loaded with indo-1/AM, angiotensin II (0.1 nM–0.1 µM) exerted a positive inotropic effect with a significant increase in the amplitude of Ca²⁺ transients. For a given increase in cell shortening, the increase in Ca²⁺ transients induced by angiotensin II was less than that induced by elevation of extracellular Ca²⁺ concentration ([Ca²⁺]_o) or isoprenaline, an indication that both the increase in mobilization of intracellular Ca²⁺ ions and myofibrillar sensitivity to Ca²⁺ ions contribute to the positive inotropic effect of angiotensin II. The effects of angiotensin II on Ca²⁺ transients and cell shortening were inhibited by the AT₁ receptor antagonist losartan. A Na⁺-H⁺ exchange inhibitor EIPA [5-(N-ethyl-N-isopropyl)amiloride] at 1 and 3 µM did not affect the Ca²⁺ transients and cell shortening, but it inhibited the angiotensin-II-induced responses in a concentration-dependent manner more effectively than the responses to elevation of [Ca²⁺]_o, indicating that EIPA elicited a selective inhibitory action on the effects of angiotensin II. The observation that EIPA at 10 µM abolished the positive inotropic effect of angiotensin II without a significant depression of the inotropic response to elevation of [Ca²⁺]_o supports the selective action of EIPA at the high concentration on the response to angiotensin II. A novel selective Na⁺-Ca²⁺ exchange (reverse mode) inhibitor KB-R7943, 2-[2-[4-(-nitrobenzyloxy)phenyl]ethyl] isothiourea methanesulphonate, at 0.3 and 1 µM inhibited also the responses to angiotensin II more effectively than the response to elevation of [Ca²⁺]_o; however, over the same concentration range it suppressed significantly the amplitude of Ca²⁺ transients and cell shortening. Combination of EIPA (3 µM) and KB-R7943 (0.3 µM), each of which attenuated partially the angiotensin-II-induced responses, abolished the positive inotropic effect and the increase in Ca²⁺ transients induced by angiotensin II with much less depressant effect on the responses to elevation of [Ca²⁺]_o. Thus, these ion exchange inhibitors exerted selective actions on the respective targets. The results with these selective inhibitors indicate that the activation of Na⁺-H⁺ exchanger and subsequent modulation of the activity of Na⁺-Ca²⁺ exchanger may be responsible for the increase in [Ca²⁺]_i and the myofilament Ca²⁺ sensitization induced by stimulation of AT₁ receptors by angiotensin II in rabbit ventricular myocytes.