Characterization of voltage‐dependent calcium channels expressed in Xenopus oocytes injected with mRNA from rat heart.

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Abstract
1. The properties of voltage dependent cardiac Ca channels expressed in Xenopus laevis oocytes after injection of mRNA from rat heart were investigated using the double-microelectrode voltage-clamp technique. 2. Endogenous Ba current (IBa, E) and expressed cardiac Ba current (IBa, C) were studied at various external concentrations of barium (Ba2+). These two entities could be distinguished by their amplitude and their pharmacology. IBa, C was more sensitive to the inorganic Ca channel blocker manganese (Mn2+). The contaminant IBa, E presented properties of voltage dependence identical to IBa, C, but was negligible in the presence of a low external Ba2+ concentration (2 mM). 3. In 2 mM-Ba2+, IBa, C activated at -35 mV, peaked at -14 mV, and reversed at +26 mV. Steady-state inactivation properties, in consideration of the half-inactivation potential of -35 mV, were also typical of L-type Ba currents. However, the decay of IBa, C was very slow (time constant of inactivation near 600 ms). No evidence for the expression of cardiac transient Ca channels (T-type) was found. 4. IBa, C was enhanced after exposure to the 1,4-dihydropyridine (DHP) agonist Bay K 8644. The enhancement of IBa, C was voltage dependent (maximum at -30 .+-. 5 mV) and associated with a slowing in current decay. Current-voltage and concentration-response curves obtained for various Ba2+ concentrations revealed an antagonism between external Ba2+ and the 1,4-DHP agonist Bay K 8644. Similar results were found using the (-)Bay K 8644 pure agonist isomer. 5. We conclude that oocytes injected with mRNA from rat heart expressed only the high threshold, long-lasting or L-type Ca channels. The availability of expressed L-type Ca channels for quantitative pharmacological studies using low Ba2+ concentration has been demonstrated.