AIMTo study and compare the excitation-contraction coupling triggered by L-type calcium current and by reverse-mode Na/Ca exchange during depolarizing steps in single guinea-pig ventricular myocytes.

METHODSWhole-cell membrane-potential, membrane-current and cell-shortening data were simultaneously acquired during whole-cell voltage clamp protocols. Voltage clamp pulses elicited ICa(L) at + 10 mV, + 50 mV, + 100 mV and evoked contractions in myocytes superfused with Tyrode's solution at 35 degrees C.

RESULTSThe greater the inhibition of I(Ca(L)), the more likely contractions would be abolished at +10 mV test potential. There was a correlation between them. At potential positive to + 50 mV, contractions were partially suppressed by Nif 100 micromol/L or Nif 30 micromol/L plus Cd2+30 micromol/L. The residual contraction was significantly delayed in onset. At +100 mV test potential, contractions were delayed in onset compare to + 50 mV and resistant to Nif 100 micromol/L or Nif 30 micromol/L plus Cd2+30 micromol/L. The residual contraction was completely blocked by Ni2+ at + 50 mV and + 100 mV.

CONCLUSIONSI(Ca(L)) is the major trigger for excitation-contraction coupling. Na/Ca exchange modulates excitation-contraction coupling as both reverse and forward mode.

Animals ; Calcium ; metabolism ; Calcium Channels, L-Type ; metabolism ; Cell Line ; Guinea Pigs ; Heart Ventricles ; cytology ; Myocardial Contraction ; physiology ; Myocytes, Cardiac ; cytology ; metabolism ; physiology ; Patch-Clamp Techniques ; Sodium ; metabolism ; Sodium-Calcium Exchanger ; physiology

AIMTo study whether the mAchR agonist Carbachol(Cch, nonselective) causes increased contractions and L-type Ca2+ current [L(Ca(L))] in ventricular myocytes and the mechanism of these effects.

METHODSThe effect of Cch on the I(Ca(L)) and Na/Ca exchange current (I(Na/Ca) was studied in patch-clamped ventricular myocytes isolated from rat hearts and superfused with Tyrode's solution (35 degrees C, 1.8 mmol/L [Ca2+]o) and stimulated at 0.2 Hz and 1.0 Hz evoke contractions of single myocyte.

RESULTS(1) An increase of stimulation frequency decreased the contractions of myocytes(negative inotropic effects). (2) 100 micromol/L Cch increased contraction in 6 cells by 28% (delta L0.2 Hz/ delta L1.0 Hz > or = 1.25) at 1.0 Hz stimulus frequency. (3) The mAchR antagonist Atropine prevented the Cch effect. The mAchR agonist McN-A-343 (M1-selective) did not change the contractions in most of the cells. (4) 100 micromol/L Cch had no significant effect on basal I(Ca(L)), but increased the tail current density on repolarization from +10 mV to -40 mV, suggested that Cch increased I(Na/Ca).

CONCLUSIONThe increase of cell contractions by Cch is apparently mediated by M2 mAchR and eventually increased I(Na/Ca). The increase Ca2+ content of the SR is reflected by the greater magnitude of I(Na/Ca). These results provide an explanation for the increased contraction of the rat ventricular myocytes by Cch and without changes in I(Ca(L)).

Animals ; Calcium Channels, L-Type ; physiology ; Carbachol ; pharmacology ; Cholinergic Agonists ; pharmacology ; Heart Ventricles ; cytology ; Muscle Contraction ; drug effects ; Myocytes, Cardiac ; physiology ; Patch-Clamp Techniques ; Rats ; Sodium-Calcium Exchanger ; physiology