ALTERED PHARMACOLOGICAL RESPONSIVENESS OF REDUCED L-TYPE CALCIUM CURRENTS IN MYOCYTES SURVIVING IN THE INFARCTED HEART

The pharmacologic responses of macroscopic L-type calcium channel curr ents to the dihydropyridine agonist, Bay K 8644, and beta-adrenergic r eceptor stimulation by isoproterenol were studied in myocytes enzymati cally dissociated from the epicardial border zone of the arrhythmic 5- day infarcted canine heart (IZs). Calcium currents were recorded at 36 degrees to 37 degrees C using the whole cell, patch clamp method and elicited by applying step depolarizations from a holding potential of -40 mV to various test potentials for 250-msec duration at 8-second in tervals. A Cs+-rich and 10 mM EGTA-containing pipette solution and a N a+-and K+-free external solutions were used to isolate calcium current s from other contaminating currents. During control, peak I-Ca,I-L den sity was found to he significantly less in IZs (4.0 +/- 1.1 pA/pF) tha n in myocytes dispersed from the epicardium of the normal noninfarcted heart (NZs; 6.5 +/- 1.8 pA/pF), Bay K 8644 (1 mu M) significantly inc reased peak I-Ca,I-L density 3.5-fold above control levels in both NZs (to 22.5 +/- 6.2 pA/pF; n = 7) and IZs (to 12.8 +/- 3.0 pA/pF; n = 5) , yet peak I-Ca,I-L density in the presence of drug was significantly less in IZs than NZs. The effects of Bay K 8644 on kinetics of current decay and steady-state inactivation relations of peak I-Ca,I-L were s imilar in the two cell types. In contrast, the response of peak L-type current density to isoproterenol (1 mu M) was significantly diminishe d in IZs compared to NZs regardless of whether Ba2(-) or Ca2+ ions car ried the current. Thus, these results indicate an altered responsivene ss to beta-adrenergic stimulation in cells that survive in the infarct ed heart. Furthermore, application of forskolin (1 mu M and 10 mu M) o r intracellular cAMP (200 mu M), agents known to act downstream of the beta-receptor, also produced a smaller increase in peak I-Ba density in IZs versus NZs, suggesting that multiple defects exist in the beta- adrenergic signaling pathway of IZs. In conclusion, these studies illu strate that reduced macroscopic calcium currents of cells in the infar cted heart exhibit an altered pharmacologic profile that has important implications in the development of drugs for the diseased heart.