The effects of extracellular calcium removal on sino-atrial node cells treated with potassium-depleted solutions.

The effect of extreme calcium reduction was studied in rabbit sino-atrial node cells treated with K-free media. Ca removal (Ca = 0, EGTA = 6 mM) suppressed spontaneous miniature fluctuations elicited by exposure to the K-free media. Further, successive exposure to the Ca-free, EGTA solutions produced a propagated action potential and suppressed a contracture. The action potential was not noticeably altered by tetrodotoxin (TTX). In the presence of normal Ko, reducing the bathing calcium concentration with EGTA prolonged the duration of the action potential. The potential induced by Ca removal was similar to that observed in the K-free media. From these results, we have concluded that a drastic reduction in the extracellular Ca concentration produces a decrease in the overloaded Ca in the cell exposed to the K-free media, resulting in an increase in the amplitude of action potentials.