A novel secretagogue increases cardiac contractility by enhancement of L-type Ca2+ current

N′1-(3,3,6,8-tetramethyl-1-oxo-1,2,3,4-tetrahydronaphthalen-2-yliden)-2-cyanoethanohydrazide (TTYC) increases secretion of glucagon-like peptide-1 and intracellular Ca2+ concentration in GLUTag cells. The purpose of the present study was to examine if TTYC exerts positive inotropic effects on isolated rabbit ventricular myocytes and in vivo heart in anesthetized rats, and if so to further define the potential mechanism of action. Contractility was assessed in vitro using changes in fractional shortening (FS) of myocyte sarcomere length and in vivo using changes in the velocity of left ventricular pressure. Changes in L-type Ca2+ current of ventricular myocytes were evaluated using whole-cell voltage-clamp techniques. TTYC increased FS of myocyte sarcomere length in a concentration-dependent manner. The positive inotropic effect was not abrogated by β-adrenergic blockade (propranolol) or protein kinase A inhibition. TTYC enhanced peak L-type Ca2+ current in a voltage-dependent manner (current amplitudes increased by 4.0-fold at -10 mV and 1.5-fold at +10 mV). Voltage-dependence of steady-state activation of L-type Ca2+ current was shifted by 15 mV in the negative direction. Inactivation time course of the L-type Ca2+ currents at voltages of −10 to 20 mV was significantly slowed by 0.3 μM TTYC. In vivo studies demonstrated that TTYC increased cardiac contractility in a dose-dependent manner. In conclusion, TTYC is a novel L-type Ca2+ current activator with positive cardiac inotropic effects. Negative shifting of the voltage-dependence of L-type Ca2+ current activation and reduced inactivation are two mechanisms responsible for the enhanced L-type Ca2+ current that contribute to the positive inotropic effects.

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