Use-dependent facilitation and depression of L-type Ca2+ current in guinea-pig ventricular myocytes: modulation by Ca2+ and isoprenaline

Objective: An increase in stimulation frequency can facilitate or depress c ardiac Ca2+ current (I-Ca). The aim was to examine the Ca2+ dependence of t hese effects, to determine if facilitation is sustained, and to elucidate t he mechanism by which isoprenaline modulates facilitation. Methods: We exam ined the effects of increasing the stimulation frequency for 1 min, from 0. 05 to 1 Hz, on I-Ca recorded from guinea-pig ventricular myocytes, using th e whole-cell, voltage-clamp technique. Results: 1 Hz stimulation caused a f acilitation of I-Ca that peaked in 5 s and was followed by depression towar ds the basal level. Metabolic inhibitors or replacement of extracellular Ca 2+ with Ba2+ abolished facilitation without affecting depression, implying that they are independent processes and that facilitation required ATP and Ca2+. Subtraction of the depression observed in either condition, from the response to 1 Hz stimulation recorded under control conditions, revealed th at I-Ca facilitation was well maintained during 1 Hz stimulation. Increased intracellular Ca2+ buffering reduced both phases of the response. Furtherm ore, varying the extracellular Ca2+ concentration ([Ca2+](o)) revealed a Ca 2+-dependent enhancement of depression and a bell-shaped dependence of faci litation on [Ca2+](o). Facilitation increased with [Ca2+](o) up to 1 mM, th en declined at higher concentrations due to partial masking by the overlapp ing depression. Isoprenaline produced concentration-dependent inhibition of facilitation and enhancement of depression when pipettes contained 2 mM EG TA, but not BAPTA. For an equivalent increase in I-Ca amplitude, the effect s of isoprenaline and elevated [Ca2+](o) on the response to 1 Hz stimulatio n were quantitatively the same. Conclusions: Facilitation is sustained duri ng increased activity, but appears transient due to overlapping depression. Both responses are promoted by increased submembrane [Ca2+]. Isoprenaline appears to modulate facilitation and depression as a consequence of increas ed Ca2+ influx, rather than cAMP-dependent phosphorylation. The apparent bl ock of facilitation by isoprenaline may result from masking by the enhanced depression. (C) 1999 Elsevier Science B.V. All rights reserved.