T-Type and Tetrodotoxin-Sensitive Ca 2+ Currents Coexist in Guinea Pig Ventricular Myocytes and Are Both Blocked by Mibefradil

Abstract

—Under Na⁺-free conditions, low-voltage–activated Ca²⁺ currents in cardiomyocytes from various species have been described either as Ni²⁺-sensitive T-type 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-cell 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 these low-voltage–activated Ca²⁺ currents, which are separated pharmacologically by superfusion with TTX or Ni²⁺. I_Ca(T) and I_Ca(TTX) were of similar amplitude but significantly differed in their electrophysiological properties: I_Ca(TTX) activated at more negative potentials than did I_Ca(T), the potential 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 the T-type Ca²⁺ channel blocker mibefradil (10 μmol/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 myocytes and that both currents are sensitive to mibefradil. Future investigations of I_Ca(T) will have to consider the TTX-sensitive current component to avoid possible interference.