Soluble substances released from postischemic reperfused rat hearts reducecalcium transient and contractility by blocking the L-type calcium channel

OBJECTIVES This study was designed to investigate the effects of cardiodepr essant substances released from postischemic myocardial tissue on myocardia l calcium-regulating pathways. BACKGROUND We have recently reported that new cardiodepressant substances a re released from isolated hearts during reperfusion after myocardial ischem ia. METHODS After 10 min of global ischemia, isolated rat hearts were reperfuse d and the coronary effluent was collected for 30 s. We tested, the effects of the postischemic coronary effluent on cell contraction, Ca2+ transients and Ca2+ currents of isolated rat cardiomyocytes by applying fluorescence m icroscopy the whole-cell, voltage-clamp technique. Changes in intracellular phosphorylation mechanisms were studied by measuring tissue concentrations of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphat e (cGMP), as well as activities of cAMP-dependent protein kinase (cAMP-dPK) and protein kinase C (PKC). RESULTS The postischemic coronary recent, diluted with experimental buffer, caused a concentration dependent reduction of cell shortening and Ca2+ tra nsient in the field-stimulated isolated cardiomyocytes of rats, as well as a reduction in peak L-type Ca2+ current in voltage-clamped cardiomyocytes. The current reduction resulted from reduced maximal conductance-not from ch anges in voltage- and time-dependent gating of L-type Ca2+ channel. The pos tischemic coronary effluent modified neither the tissue concentrations of c AMP or cGMP nor the activities of cAMP-dPK and PKC. However, the effluent c ompletely eliminated the activation of glycogen phosphorylase after beta-ad renergic stimulation, CONCLUSIONS Negative inotropic substances released from isolated postischem ic hearts reduce Ca2+ transient and cell contraction through cAMP-independe nt and cGMP-independent blockage of L-type Ca2+ channels. (C) 2001 by the A merican College of Cardiology.