L-type Ca21 current and excitation-contraction coupling in single atrial myocytes from rainbow trout

Hove-Madsen, Leif, and Lluis Tort. L-type Ca21 current and excitation-contraction coupling in single atrial myocytes from rainbow trout. Am. J. Physiol. 275 (Regulatory Integrative Comp. Physiol. 44): R2061–R2069, 1998.—We have examined the contribution of L-type Ca21 current (ICa) to the activation of contraction in trout atrial myocytes under basal and phosphorylating conditions. The average myocyte length was 197 6 14 μm, width was 5.5 6 0.2 μm, and cell capacitance was 36.2 6 2.2 pF. With 25 μM EGTA in the patch pipette and a stimulation frequency of 0.125 Hz, ICa was 2.6 6 0.4 pA/pF and it carried a total charge of 0.10 6 0.01 pC/pF, giving rise to a contraction of 15.2 6 2.8% of the resting cell length. With a cell volume of 2.4 6 0.3 pl, the charge carried by ICa corresponded to 14.7 6 2.2 μmol Ca21/l nonmitochondrial cell volume (μM). This can account for only 30–40% of the Ca21 binding to the myofilaments during a contraction. Increasing the stimulation frequency from 0.25 to 2 Hz decreased ICa amplitude and charge by 66 6 5 and 80 6 3%, respectively. Elevating the pipette EGTA concentration from 25 μM to 5 mM increased ICa amplitude and charge by ,290%. Both isoproterenol and cAMP increased ICa by ,230%. The total charge carried by the isoproterenolor cAMPstimulated current was increased by 170%. We conclude that the use of high-EGTA concentration may overestimate the total Ca21 carried by ICa under physiological conditions. Furthermore, the results suggest that, in contrast to previous reports from other lower vertebrates, Ca21 flux through L-type Ca21 channels alone is not sufficient to fully activate contraction in trout atrial myocytes at room temperature.