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

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Abstract
—Under Na + -free conditions, low-voltage–activated Ca 2+ currents in cardiomyocytes from various species have been described either as Ni 2+ -sensitive T-type Ca 2+ current ( I Ca(T) ) or as tetrodotoxin (TTX)-sensitive Ca 2+ 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 Ca 2+ currents, which are separated pharmacologically by superfusion with TTX or Ni 2+ . 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 Ni 2+ is critical because of possible shifts in voltage dependence. Therefore, we investigated whether the T-type Ca 2+ 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.