Ion transfer characteristics of the calcium current in bull‐frog atrial myocytes.

Abstract

1. Voltage clamp studies on single cells from bull-frog atrium have been carried out to study the ion transfer characteristics of the calcium current, ICa. In agreement with the preliminary results of Hume and Giles (1983), a TTX-resistant, ''second transient inward current'' was recorded consistently. Its average peak size at 0 mV in 2.5 mM [Ca2+]o Ringer solution was approximately -200 pA, and it was blocked by Cd2+ and La3+ but not by tetrodotoxin (TTX, 3 .times. 10-6 M). 2. The peak size of this current increases by approximately 4 times when [Ca2+]o is raised from 1.25 to 7.5 mM, indicating that Ca2+ is a major charge carrier. 3. A well-defined reversal potential, Erev, for ICa can be recorded in normal Ringer solution and also when Ba2+ or Sr2+ serve as the charge carriers. When [Ca2+]o is changed the shifts in Erev follow the predictions of a Nernstian Ca2+ electrode. However, all Erev values are well below those predicted from the thermodynamic Nernstian ECa (see Campbell, Giles, Hume, Noble and Shibata, 1988a). 4. The Ca2+ current exhibits voltage-dependent inactivation, whether the direction of net current flow is inward or outward; however, the rate of inactivation is affected by the species of cation carrying the current. Inactivation is reduced substantially in Ba2+ Ringer solution. 5. Magnesium (5 mM) is not a significant carrier or blocker of ICa in normal [Ca2+]o Ringer solution; however, 5 mM [Mg2+]o can block the current carried by either Sr2+ or Ba2+. In the absence of Mg2+, equimolar substitutions of Sr2+ or Ba2+ for Ca2+ result in larger currents than those carried by Ca2+ in the normal Ringer solution. 6. Sodium appears not to be a significant charge carrier in the presence of normal [Ca2+]o. However, after free [Ca2+]o has been reduced to extremely low levels (< 10-6 M) Na+ can carry a significant fraction of ''ICa''. Thus, it appears that the high selectivity of ICa for Ca2+ ions depends upon the presence of Ca2+. 7. ''Slow tails'' are frequently recorded after repolarizing clamp step back to the holding potential. These ''slow tails'' are prominent in normal [Na+]o, [Ca2+]o and [Sr2+]o Ringer solution; however, they are markedly reduced in [Ba2+]o, in Na+-free and Ca2+-free Ringer solutions. Experimental and theoretical work suggests these slow tails may be generated by an electrogenic Na+-Ca2+ exchanger (see Campbell, Giles, Robinson and Shibata, 1988b). 8. The ion transfer properties of ICa in myocytes from bull-frog atrium are compared and contrasted with those of ICa in other cardiac myocytes, and the functional significance of this current is discussed.