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Short-term (10 min) effects of 100 nM 12-O-tetradecanoylphorbol-13-acetate (TPA), the protein kinase C (PKC) activator, on cardiac macroscopic (gj) and single channel (gamma j) gap junctional conductances were studied in pairs of neonatal rat cardiomyocytes. Under dual whole-cell (WC) or perforated patch (PP) voltage-clamp, gj increased by 15.5 +/- 7.2% (mean +/- SD, n = 9) and by 46.3 +/- 17.0% (n = 5), respectively. The latter difference is not related to intracellular calcium concentration, because raising the Ca2+ concentration in the electrode solution did not change the TPA-induced increase in gj observed under WC conditions. The inactive phorbol ester, 4 alpha-phorbol 12,13-didecanoate (alpha PDD), did not affect gj. Single cardiac gap junction channel events, resolved in the presence of heptanol, indicated two gamma j sizes of 20 and 40-45 pS. Under control conditions, the larger events were most frequently observed. Whereas alpha PDD did not change this distribution, TPA shifted the gamma j distribution to the lower sizes. Diffusion of Lucifer Yellow (LY) and 6-carboxyfluorescein (6-CF), gap junction permeant tracers, was studied on small clusters of cardiomyocytes. Under control conditions, LY labeled 19.4 +/- 7.2 cells (mean +/- SD, n = 18) and 6-CF labeled 8.4 +/- 2.2 cells (n = 20). Whereas alpha PDD did not change the extent of dye transfer, TPA restricted the diffusion of LY to 2.8 +/- 1.3 cells (n = 11) and of 6-CF to 2.4 +/- 1.4 cells (n = 20). This suggests that permeability and single channel conductance of connexin 43 channels are parallely related. Altogether, these results point to the opposite modulation of electrical and metabolic coupling of cardiac cells evoked by TPA.