Dual effects of dihydropyridines on whole cell and unitary calcium currents in single ventricular cells of guinea-pig.

We studied the effects of dihydropyridine Ca channel ligands (DHPs), mainly nitrendipine and Bay K8644, on whole cell and single channel Ca currents on single myocytes isolated from the adult guinea-pig ventricle. Nitrendipine had dual effects, stimulatory or inhibitory, depending upon the membrane potential. At low frequencies (less than 0.03 Hz) and negative holding potentials (-90 mV or more), nitrendipine increased the Ca currents in a dose-dependent manner. The dose-response curve was best fitted by a Langmuir adsorption isotherm model which was the sum of two independent one-to-one drug-receptor sites with median effective doses (ED50S) of 1.0 X 10(-9) M and 1.4 X 10(-6) M respectively. When the membrane potential was held at -30 mV or less, nitrendipine inhibited the Ca currents, also in a dose-dependent manner. The dose-response curve was fitted by a single binding site model having a median inhibitor concentration (IC50) of 1.5 X 10(-9) M. At holding potentials between -70 and -40 mV, nitrendipine produced mixed effects on Ca currents; an increase occurred initially and this was followed by a decrease. When rundown was excluded, Bay K8644 showed only stimulatory effects on the Ca currents between holding potentials of -120 and -30 mV. When the test potential was zero or +10 mV the Ca currents reached peak values and the dose-response curve was best fitted by a single binding site model having an ED50 of 3 X 10(-8) M. When the effects were measured at negative test potentials of -30 to -10 mV, the curve was best fitted by a two-site model with ED50S of 3 X 10(-9) and 9 X 10(-7) M. At the single Ca channel level the stimulatory effect of nitrendipine was due to an increased probability that a Ca channel which had opened once would reopen, a reduction in records without activity and an increase in the mean open time. There were no changes in unit conductance. Inhibitory effects were due to a large increase in nulls. At lower concentrations the main effect of Bay K8644 was an increase in the probability of opening. At doses above 10(-6) M, a pronounced increase in the open time was observed. The effects we observed are attributed to at least two sites for DHP related to Ca channels; one with high affinity and one with a lower affinity. The low affinity site mediates a stimulatory effect due to greatly prolonged openings.(ABSTRACT TRUNCATED AT 400 WORDS)