COMMENTARY

Ca-activated K channels: the ins and outs of calmodulin

FERNANDO ORREGO

Laboratorio de Neurociencias, Facultad de Medicina,
Universidad de Los Andes, Santiago-20, Chile

The presence in human red blood cells (RBCs) of a K permeability that was greatly enhanced by calcium was first reported by Gardós (1958). It was soon shown that this effect depended on an increase in intracellular calcium (Cai2+) (Whittam, 1968), and that the calcium sensor was located at the inside of the membrane (Blum and Hoffman, 1972). The nature of this Ca sensor or receptor was unknown however, but due to its differential selectivity for divalent cations, Meech (1976) hypothesized that it could be related to muscle troponin c. Later experiments, in which six different calmodulin-inhibitory drugs were tested on the Ca-activated K conductance of human RBCs, showed that all were potent inhibitors of this conductance and that their effect showed a correlation coefficient of 0.98 with that found on RBC Ca-ATPase, a membrane enzyme known to be activated by calmodulin (Lackington and Orrego, 1981). More importantly still, the blocking effect of these drugs on Ca-activated K conductance was unrelated to their hydrophobicity or to their ability to penetrate RBC lipid monolayers (Orrego et al., 1985). In the following years it was shown that Ca, in conjunction with calmodulin, activated this type of K channels in RBC inside-out vesicles (Pape and Kristensen, 1984), in adipocyte membranes (Pershadsingh et al., 1986), and in fibroblasts (Okada et al., 1986), while in Paramecia, a mutant calmodulin led to the loss of Ca-activated K conductance, which could be restored by injection of wild type calmodulin (Schaefer et al., 1987). All these findings thus seemed to firmly support a role for calmodulin in the gating of these Ca-activated K channels. These conclusions were, however, apparently contradicted by elegant experiments done on internally-perfused snail neurons, in which neither the injection of calmodulin, nor of the calmodulin-antagonist, trifluoperazine, were able to modify Ca-activated K currents (Levitan and Levitan, 1986). At the time, this seemed to tip the balance against a participation of calmodulin in these channels.

In the early studies, no clear distinction was made between the Ca-activated K currents seen in RBCs and those present in a large variety of other tissues and species in which these channels began to be described (Meech, 1976; Putney, 1979; Schwarz and Passow, 1983). This was better defined when recordings were made of single channels in many different preparations in combination with the use of selective blockers. Based on differences in unitary conductances, sensitivity to drugs, and voltage-dependence, this allowed the classification of the channels as small (SK), with unitary conductances of 4 - 14 pS that are blocked by apamin and are insensitive to voltage. These SK channels are abundant in the nervous system, where they mediate slow after-hyperpolarizations that modulate excitability. Intermediate (IK) channels, on the other hand, have conductances of 11- 40 pS, are voltage-independent, may be blocked by iberiotoxin and charybdotoxin (Ch Tx), and are found in red and white blood cells, colon, lung, pancreas, and other tissues. Large conductance (BK) channels, on the other hand, have unitary conductances of 100 - 260 pS, show a marked voltage dependency that is modulated by Ca2+i , are also blocked by ChTx, and are found in muscle, neurons and chromaffin cells, where they were first described (Marty, 1981; Pallotta et al., 1981; Latorre et al., 1982; Adams et al., 1982; Latorre et al., 1989).

Very recently, the sequencing of the genes that code for SK, IK and BK channels has allowed a definition of the mechanisms of channel gating and of the participation of calmodulin in it. Thus, SK and IK channel proteins lack Ca2+ - binding motifs, but have calmodulin-binding domains near their intracellular carboxy-terminal regions to which calmodulin binds tightly and confers Ca-sensitivity to the channel (Xia et al., 1998; Fanger el al., 1999). On the other hand, BK channels have an aspartate-rich Ca-sensing region (a «calcium bowl») in the channel protein itself, which confers most of the Ca-sensitivity to the channel, although a second Ca-sensor also seems to be present (Schreiber and Salkoff, 1997). These findings are, therefore, of interest because they confirm the earlier indirect suggestion that calmodulin indeed plays a role in the gating of Ca-activated K channels of RBCs (Lackington and Orrego, 1981) and other tissues (Pape and Kristensen, 1984; Pershadsingh et al., 1986; Okada et al., 1986), as well as in Paramecia, where a direct demonstration of the participation of calmodulin had already been offered (Schaefer et al., 1987). On the other hand, these recent studies apparently also support the lack of involvement of calmodulin in the gating of the BK channels studied in Helix neurons by Levitan and Levitan (1986).

The participation of calmodulin, however, is not restricted to the regulation of Ca-activated K channels, as numerous other studies have shown that it also participates in the feed-back inhibition by Ca of the L-type of Ca channels (Qin et al., 1999) and of the Ca-selective NMDA receptor channels (Zhang et al., 1998) and that it also inhibits the activity of cyclic nucleotide-gated channels in the visual and olfactory systems (Levitan, 1999). This vast realm of calmodulin's channel regulatory functions was certainly unsuspected when we first suggested its participation in RBC Ca-activated K conductance in 1981.

ACKNOWLEDGEMENTS

We are grateful to CONICYT and FONDECYT for their support over the past 30 years and to Universidad de Chile Project B 396-8144, which financed our original work in this field.

Corresponding author: Fernando Orrego, e-mail: neuroc@uandes.cl

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