Contribution of two types of calcium currents to the pacemaker potentials of rabbit sino-atrial node cells

1. Two types of calcium currents, the transient type and long-lasting type, were examined by both whole-cell and cell-attached patch-clamp modes in single isolated sino-atrial node cells of the rabbit. 2. In the whole-cell clamp mode, in response to a depolarizing pulse to -40 mV from a holding potential of -80 mV, a transient type calcium current with an amplitude of 2.1 +/- 0.7 pA/pF (mean +/- S.D.; n = 15) was recorded. The threshold potential was approximately -50 mV. 3. Nickel (40 microM) and tetramethrin (0.1 microM) blocked the transient type calcium current without appreciable effects on the long-lasting type. Nifedipine and D600 blocked the long-lasting type, but did not affect the transient type. Cadmium (20 microM) and cobalt (2 mM) inhibited both types of calcium currents equally. 4. Both types of calcium currents showed an increased amplitude with increasing extracellular calcium concentration. The values of the Michaelis constant, Km, were 0.95 mM for the transient type and 3.92 mM for the long-lasting type, indicating that these types represent two different classes of channels. 5. In the cell-attached patch-clamp mode, the single-channel conductance of the transient type calcium current was 8.5 pS, by using 100 mM-BaCl2 in the pipette, whereas that of the long-lasting type was 16.0 pS, under the same conditions. Each of these values was similar to those found in other cells, respectively. 6. In the whole-cell clamp mode, the transient type current began to inactivate at -70 mV and was fully inactivated at -40 mV. The steady-state inactivation curve of the transient type current was approximately 50 mV negative to that of the long-lasting type. The overlap of the membrane potential between the activation and inactivation curves was small. The time constant of the inactivation shortened from 20 to 5 ms as the potential became progressively positive over the range from -80 to +30 mV. 7. Isoprenaline (1 microM) increased the amplitude of the long-lasting type Ca2+ current, but was not effective on the transient type, suggesting that the long-lasting type calcium current may be responsible for the positive chronotropic effect of isoprenaline. 8. While recording spontaneous electrical activity of the cell, application of 40 microM-nickel induced bradycardia and this effect was enhanced when the membrane was constantly hyperpolarized.(ABSTRACT TRUNCATED AT 400 WORDS)