Effects of ethylene glycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid on rabbit sinoatrial node cells treated with cardiotonic steroids.

The effect of ethylene glycol-bis-(beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) was studied in rabbit sinoatrial node cells treated with ouabain. When isolated sinoatrial node preparations were bathed in the presence of 10(-5) M ouabain in order to depress electrogenic Na-K pump activity, action potentials ceased rapidly. Thereafter, spontaneous miniature fluctuations of less than 2 mV were observed at the resting potential level, while the membrane potential reached a value of about -31 mV. After exposure to the Ca-free 2 mM EGTA solutions, the miniature fluctuations decreased in frequency as well as in amplitude. However, successive exposure to the Ca-free EGTA solutions produced a short series of slow regenerative potentials. Peak amplitude of the slow regenerative potential increased from reduced miniature fluctuations to a maximum value of 46 +/- 3 mV (mean +/- S.E.; n = 10). Resting potential before initiation of the slow regenerative potential reached a value of about -34 mV. Also, Ca-free perfusion produced a small regenerative potential reaching 19 +/- 2 mV (n = 3). In Ca-free EGTA solution, reduction of external Na produced a decrease in amplitude of the slow regenerative potential when choline chloride or LiCl replaced NaCl. Replacement of 25% of the external Na with choline suppressed the slow regenerative potential. Replacement of 60% of the external Na with Li did not affect the slow regenerative potential. Further reduction of the external Na to 0% of the control condition produced a small regenerative potential. Replacement of Na with Li did not abolish the regenerative potential. In the presence of 4 x 10(-6) M verapamil Ca-free EGTA perfusion did not induce a slow regenerative potential. In the presence of 0.7 x 10(-6) M nifedipine Ca-free EGTA perfusion induced a small regenerative potential reaching 27 +/- 5 (n = 3). For low external pH (6.3) obtained with 5% CO2, the slow regenerative potential induced by Ca-free, EGTA perfusion was suppressed, and the regenerative potential amplitude decreased by about 16 mV. Application of 2.5 mM SrCl2 caused a decrease in slow regenerative potential, and the potential decreased by about 21 mV. The slow regenerative potential is not altered noticeably by 10(-5) M tetrodotoxin.(ABSTRACT TRUNCATED AT 400 WORDS)