Maturation and myotonia influence the abundance of cation channels K DR , K IR and C IR differently: a patch-clamp study on mouse interosseus muscle fibres

Abstract

To detect cation channels, the expression of which is dependent on the physiological state of muscle, single-channel activities of dissociated fibres of the mouse interosseus muscle were recorded using the patch-clamp technique in the cell-attached mode. Fibres were prepared from juvenile and adult wild-type (WT), from chloride channel-deficient myotonic and from denervated adult WT muscles. In all cases delayed-rectifier K + channels (K DR ) with a unitary conductance of 11 pS were recorded in more than 95% of sarcolemmal patches, but with a low, steady-state open probability. Inwards-rectifying K + channels (K IR ) with a conductance of 31 pS in 140 mM [K + ] o were active in about 50% of the membrane patches from WT and in more than 90% of those from myotonic fibres. A hitherto undescribed, inwards-rectifying, cation channel, provisionally termed C IR , with fast kinetics and a unitary conductance of 36 pS, was active in nearly every membrane patch from juvenile mice, both WT and myotonic. The abundance of C IR decreased during development, but was not changed 7 days after denervation of adult WT muscle. Ca 2+ -dependent K + channels were seen sporadically. Channels with the characteristics of adenosine 5'-triphosphate (ATP)-sensitive K + channels were recorded frequently upon excision of membrane patches, but remained inactive in most cell-attached recordings. In conclusion, of the investigated ion channels, only K IR was responsive to the activity pattern of adult muscle, whereas C IR was down-regulated during muscle maturation.

Isoprenaline-stimulated differential adrenergic response of K+ channels in skeletal muscle under hypokalaemic conditions

“…Firstly, the cellattached experiments infer the functional presence of K IR and BK channels in the sarcolemma of skeletal muscle, where K IR dominates P K under control conditions. Channel characterization is consistent with reports in literature for K IR [5,19,21,22,23,24,26,31] and BK [2,25,29,36] channels. Additionally, due to the circumstance that the activity of K IR decreases and that of BK increases following application of Iso under identical experimental conditions, it is concluded that the effect of the increased BK contribution induced by Iso exceeds the concomitant effect of the decrease of the K IR .…”

“…The current records (Fig. 1A) displayed long-lasting stable open intervals typical for K IR channels [19,23,26]. Sometimes sublevel currents at approximately 80% of maximum value were found [26].…”

“…The corresponding V 1/2 was 27 mV (Table 1). These i/V characteristics, open-closed behaviour and the voltage dependence of P open are hallmarks for the K IR channel [5,19,21,22,23,24,26,31].…”

“…1B). Once an K IR channel was found, V rev was identified as the Nernst (equilibrium) potential for K + (E K ), since it is well known that the potential at which the K IR channel stops conducting is E K [16,18,19,22,23,26,31]. On the other hand, when g exceeded 100 pS and no clear rectification was found at V rev , the channel was identified as a BK channel (see Fig.…”