The effects of external Ca++ and Mg++ on the voltage sensitivity of desensitization in Electrophorus electroplaques.

Abstract

Desensitization onset was studied in voltage-clamped Electrophorus electroplaques during prolonged exposure to bath-applied carbamylcholine. The time-course of desensitization was described by a first-order rate constant ko~, which increased exponentially with membrane hyperpolarization from -20 to -90 inV. When Ca ++ was increased from 2 to 10 mM, the voltage sensitivity of ko~ decreased; ko~ decreased for voltages more negative than -40 mV, and increased slightly at voltages more positive than -40 mV. 10 mM Mg ++ had a less pronounced effect and the voltage sensitivity of koh was unchanged. The equilibrium level of desensitization, estimated from the carbamylcholine-dependent conductance which remained after desensitization was apparently complete, also increased with hyperpolarization.

Desensitization of the acetylcholine receptor of frog end‐plates measured in a Vaseline‐gap voltage clamp.

“…It is unlikely that the voltage dependence reported here is secondary to the voltage dependence of calcium entry, as has been suggested by Miledi (1980), Chesnut (1983) The effect of calcium There have been many reports that calcium can increase the rate of desensitization (e.g. Manthey, 1966Manthey, , 1970Fiekers et al 1980;Miledi, 1980), though a decreased rate has been seen in Electrophorus (Pallotta & Webb, 1980). With 80 mM-K2EGTA as the intracellular solution we observed that the time course of desensitization was unaffected by changing the extracellular calcium concentration from 1-5 mm to 20 /M, so calcium is certainly not necessary for desensitization to occur.…”

“…There is relatively little quantitative information on the extent of voltage dependence at end-plate receptors; Fiekers et al (1980) found an e-fold increase in the single desensitization rate constant that they measured (roughly comparable to our faster rate) for a hyperpolarization of 83 mV with 250 ,uM-carbachol (which is roughly equipotent with 15-25 ,tm-ACh), in potassium-depolarized frog muscle at room temperature with 1-3 mM-external calcium concentration (there was little effect of calcium concentration on the voltage dependence). In Electrophorus Pallotta & Webb (1980) found a greater voltage dependence, an e-fold increase in rate for a hyperpolarization of 33 mV in the single rate constant that they observed. We observed, in fibres containing 80 mm-K2EGTA, a more modest voltage dependence in 'r, an e-fold increase in rate corresponding to a 100-200 mV hyperpolarization, and we were unable to detect a consistent voltage dependence in the slow component though, if anything, this also became a bit faster on hyperpolarization.…”

“…This result is, of course, also consistent with the view that ACh is capable, in the absence of desensitization, of opening a large proportion of channels, the parameters of the peak response versus concentration curve being consistent with those found by Colquhoun & Ogden (1988) (see also discussion of mechanisms below). Depcndence of the rate of nicotinic receptor desensitization on membrane potential has been reported in frog end-plate by Magazanik & Vyskocil (1970), Scubon-Mulieri & Parsons (1978) and by Fiekers et al (1980), and in Electrophorus electroplaques by Pallotta & Webb (1980). On the other hand nicotinic-like receptors in neurones of Lymnea 8tagnali8 (Chemeris et al 1982) and in Aplysia (Slater et al 1984) show no clear voltage sensitivity.…”

A study of desensitization of acetylcholine receptors using nerve‐released transmitter in the frog

“…A residual Ca2+ hypothesis for the desensitizable state is challenged, however, by the observation that external Ca2+ is not required for desensitization to occur at the neuromuscular junction (Fiekers et al 1980) and the findings that external Ca2+ has an inhibitory effect on desensitization in the electroplaque (Pallotta & Webb, 1980) or no apparent effect in the mollusc (Bregestovski, Bukharaeva & Iljin, 1979).…”

“…In scheme (1) desensitization is a simple consequence of channel opening such that a small fraction of the receptor-channel complexes with open channels proceed to the desensitized state at some characteristic rate (see del Castillo & Katz, 1957;Katz & Thesleff, 1957;Rang & Ritter, 1970;Weiland, Georgia, Lappa, Chignell & Taylor, 1977;Barrantes, 1978;Heidmann & Changeux, 1979a, b;Quast, Schimerlick & Raftery, 1979;Pallotta & Webb, 1980). In scheme (2) desensitization occurs as a result of agonist binding to the open-channel state of the receptor-channel complex (see Nastuk & Gissen, 1966;Rang & Ritter, 1970;Adams, 1975).…”