Voltage-dependent properties of macroscopic and elementary calcium channel currents in guinea pig ventricular myocytes

Abstract

Whole-cell Ca channel currents were recorded from guinea pig ventricular myocytes that were internally perfused with Cs solution and bathed in solutions containing 3.6 mM Ca, 3.6 mM Ba or 90 mM Ba (34 degrees C). Single Ca channel currents were recorded from cell-attached membrane patches of similar myocytes; the patch pipettes contained a 90 mM Ba solution. 1. Although the shape of the whole-cell I-V relation was independent of the bathing solution, this was not the case with the location of the inward current maximum (Vpeak); Vpeak in 90 mM Ba was about 30 mV positive to Vpeak in 3.6 mM Ba. 2. The activation and inactivation of whole-cell currents were voltage dependent. Compared to the voltage dependencies in 3.6 mM Ba, those in 90 mM Ba were shifted by about 30 mV to the right, suggesting a neutralization of surface charges. 3. Observations compatible with the ion permeation model proposed by Hess and Tsien (1984) included (a) a depression of current during Ca/Ba solution exchange, (b) a high divalent to monovalent ion permeability, and (c) rectification of the outward limb of the I-V relation. 4. Estimated current densities at Vpeak were similar for myocytes in 3.6 mM Ca and 3.6 mM Ba, and about 10 times larger in 90 mM Ba. 5. Average currents (I) calculated from ensembles of records of single Ca channel current had voltage-dependent time courses resembling those of whole-cell IBa (90 mM). 6. Single-channel I-V relations were superimposable on whole-cell I-V curves suggesting that voltage-dependent single-channel parameters (probability of opening, elementary current amplitude) can be related to the voltage-dependent macroscopic current parameters (activation, instantaneous I-V relation) when scaled by channel number. 7. The density of Ca channels in myocytes was calculated from whole-cell IBa (90 mM) and average current through single channels. The outcome, 3-5 channels/micron 2, agrees with two other recent estimates (Tsien et al. 1983; Lux and Brown 1984). However, it is difficult to reconcile with the much lower density that one would forecast from the frequency of functional channel observation in myocyte membrane patches (Pelzer et al. 1985c).

L-type cardiac calcium channels in doxorubicin cardiomyopathy in rats morphological, biochemical, and functional correlations.

“…To account for the putative compensatory increase in Ic, either the properties of the activated channels are altered and/or otherwise "hibernating" channels in the surface or the remaining t-tubular membrane have been recruited. In view of the fact that the calcium channel density as estimated by B. is uniformly several-fold larger than the number of electrically active channels determined by single channel recordings (37), there appears to be an abundance of hibernating channels in reserve, a notion that is akin to the concept of "spare" receptors.…”

Molecular Pharmacology of Voltage-Dependent Calcium Channels

“…Multiple ion occupancy of the channel pore is primarily dependent on the observation of an anomalous mole fraction effect between Ca 2+ and Ba2+ (109,(250)(251)(252), but this observation has not been fully characterized experimentally after a decade following the proposal of the hypotheses. Kuo and Hess have systematically characterized the physiochemical nature of the highaffinity binding sites and ion-ion interaction in the pore by examining the kinetics of blockade by different divalent metal cations of unitary L-type currents in rat phaeochromocytoma PC-12 cells (101,112,113).…”

Membrane currents in taste cells of the rat fungiform papilla. Evidence for two types of Ca currents and inhibition of K currents by saccharin.

“…This difference may be due to neutralization of surface charges by the high Ba or Ca concentration needed to reveal the currents (Hale, 1984;McDonald et al, 1986;Akaike et al, 1989). When the external Ca was reduced, a lowering of the threshold, concomitant with a decrease of the current amplitude, was observed (Fig.…”