AHEART August 46/2

Wasserstrom, J. Andrew, and Ana-Maria Vites. Activation of contraction in cat ventricular myocytes: effects of low Cd21 concentration and temperature. Am. J. Physiol. 277 (Heart Circ. Physiol. 46): H488–H498, 1999.—The effects of Cd21 (20 μM) and different bath temperatures were used to study the contributions of two separate triggering mechanisms, L-type Ca21 current (ICa) and reverse mode Na1/Ca21 exchange, to excitation-contraction (E-C) coupling in cat ventricular myocytes. Ionic currents and cell shortening were studied with patch pipettes filled with K1-containing internal solution and discontinuous (‘‘switch’’) voltage clamp. Superfusion with Cd21 blocked cell shortening that closely mirrored the block of ICa; the voltage dependence of Cd21-induced reduction in contraction was bell-shaped, displaying minima at test potentials below 210 mV and above 150 mV and a maximum at about 120 mV. Cd21-insensitive cell shortening was blocked by ryanodine (10 μM) and Ni21 (4–5 mM). When an action potential was used as the command waveform for the voltage clamp (action potential clamp), Cd21 reduced contraction to ,60 6 7% of control cell shortening (n 5 7). The remaining contraction was blocked by ryanodine and Ni21. Superfusion with nifedipine (10 μM) caused nearly identical effects to Cd21. The voltage dependence of contraction was sigmoidal at temperatures above 34°C but bell-shaped below 30°C. When Cd21 was added to superfusate, contraction was abolished at 25°C (to 6 6 3% of control) but reduced only modestly at 34°C (to 65 6 13% of control, test potential 110 mV, n 5 4, P , 0.01). These results indicate that 1) there is a component of contraction that is sensitive to ICa antagonists, and the block is equivalent with either organic or inorganic antagonists; 2) the contribution of Na1/Ca21 exchange to triggering of contraction under our experimental conditions is fairly linear throughout the entire voltage range tested; 3) the contribution of ICa is superimposed on this background component contributed by the Na1/Ca21 exchanger; and 4) triggering via the exchanger is temperature-dependent, providing a major contribution at physiological temperatures but failing at temperatures below 30°C in a nearly all-or-none fashion.