T-type and tetrodotoxin-sensitive Ca2+ currents coexist in guinea pig ventricular myocytes and are both blocked by mibefradil

Under Na+-free conditions, low-voltage-activated Ca2+ currents in cardiomyo cytes from various species have been described either as Ni2+-sensitive T-t ype Ca2+ current (I-Ca(T)) or as tetrodotoxin (TTX)-sensitive Ca2+ current (I-Ca(TTX)). So far, coexistence of the 2 currents within the same type of myocyte has never been reported. We describe experimental conditions under which I-Ca(T) and I-Ca(TTX) can be separated and studied in the same cell. Rat and guinea pig ventricular myocytes were investigated with the whole-ce ll voltage-clamp technique in Na+-free solutions. Whereas rat myocytes lack I-Ca(T) and exhibit I-Ca(TTX) only, guinea pig myocytes possess both of th ese low-voltage-activated Ca2+ currents, which are separated pharmacologica lly by superfusion with TTX or Ni2+. I-Ca(T) and I-Ca(TTX) were of similar amplitude but significantly differed in their electrophysiological properti es: I-Ca(TTX) activated at more negative potentials than did I-Ca(T), the p otential for half-maximum steady-state inactivation was more negative, and current deactivation and recovery from inactivation were faster. I-Ca(TTX) but not I-Ca(T) increased after membrane rupture ("run-up"). Isolation of I -Ca(TTX) by application of the bivalent cation Ni2+ is critical because of possible shifts in voltage dependence. Therefore, we investigated whether t he T-type Ca2+ channel blocker mibefradil (10 mu mol/L) is a suitable tool for the study of I-Ca(TTX). However, mibefradil not only blocked I-Ca(T) by 85+/-2% but also decreased I-Ca(TTX) by 48+/-8%. We conclude that under Na +-free conditions I-Ca(T) and I-Ca(TTX) coexist in guinea pig ventricular m yocytes and that both currents are sensitive to mibefradil. Future investig ations of I-Ca(T) will have to consider the TTX-sensitive current component to avoid possible interference.