Reversal potential of the calcium current in bull‐frog atrial myocytes.

Abstract

SUMMARY1. Voltage clamp recordings of the calcium current (ICa) in single myocytes which were enzymatically isolated from bull-frog atrium show that a genuine reversal of the current flowing through Ca2" channels can be recorded (cf. Reuter & Scholz, 1977;Lee & Tsien, 1982Campbell, Giles & Shibata, 1988c 4. The apparent reversal potential is analysed using a form of the constant field equation which has been modified to include (i) simultaneous monovalent and divalent cation movements and (ii) the presence of a surface potential (V'). This equation can be solved to yield an explicit expression for Erev. The effects of V' on apparent permeability ratios for the Ca2+ channel Erev are demonstrated.5. In combination, our experimental results and calculations suggest that: (i) previous estimates of V' which were used to describe permeability (P) ratios of Caa2+ channels in various cardiac preparations may be in error, (ii) in normal [Ca2+]0 the PNa/PCa ratio is very small, and (iii) PCa/PK must be greater than 1000. An analysis of the relative selectivity of the channel for divalent cations compared to K+ shows that PCa > Psr > PBa, assuming that PK remains the same after the divalent substitutions.6. The Ca21 channel in bull-frog atrial cells is thus much more selective for Ca21 ions than had previously been estimated; in particular, inward flow of monovalent

Identical Unitary Current Amplitude and Ca2+ Block of Cardiac Na Channel before and during .BETA.-Adrenergic Stimulation.

“…i ϭ0 mM [3,5]. This value was obtained from the measured shift in reversal potentials, based on the Nernst-Planck electrodiffusion theory or modified version of Goldman-Hodgkin-Katz equations [24]. Because this procedure is based on the assumptions of independence (i.e., ions cross-membrane without interacting with one another), the reported changes in permeability ratio cannot be directly related to changes in unitary current amplitude measured in our study, where ionic environments for both Na ϩ and Ca 2ϩ are different. ]…”

Studies of the sodium‐calcium exchanger in bull‐frog atrial myocytes.

“…A voltage clamp pulse to a given potential (usually 0 mV) was then applied. This generated an ICa 'forcing function', the magnitude and time course of which is very similar to the experimentally recorded ICa (Hume & Giles, 1983;Robinson, 1983; (Campbell, Giles & Shibata, 1988c;Campbell et al 1988a), the tetrodotoxin (TTX)-resistant transient inward current is an essentially 'pure' calcium current in normal 2-5 mm [Ca2+] Properties of the slow tails (a) Cationic dependence. Figure 3 illustrates typical currents recorded from single bull-frog atrial myocytes during superfusion with Ringer solutions having altered Na+ content, or in which the Ca2+ was replaced with Sr2+ or Ba2 .…”

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

“…No Ca2+-activated K+ current or Ca2+-activated non-specific current (e.g. Colquhoun et al 1981 (Campbell, Giles, Hume, Noble & Shibata, 1988a).…”

“…Hagiwara & Byerly, 1981) Hess & Tsien, 1984;Byerly et al 1985); (vi) the appearance of non-specific monovalent cation current only when [Ca2+]0 is reduced to submicromolar levels (cf. ; (vii) an apparent reversal potential which shifts with changes in external Ca2+, Sr2+, and Ba2+ concentrations, according to the predictions of a Nernstian divalent cation electrode (see Campbell, Giles, Hume, Noble & Shibata, 1988a); (viii) inactivating outward currents at potentials positive to Erev under all ionic conditions (cf. Lee & Tsien, 1982, 1984; (ix) the appearance of slow tails upon repolarization, with these slow tails displaying marked external cationic dependence (cf.…”