The role of K+ channels in the force recovery elicited by Na+-K+ pump stimulation in Ba2+-paralysed rat skeletal muscle

Abstract

The present experiments were performed to assess the role of K+ channels in hormonal stimulation of the Na+-K+ pump and to determine the contribution of Na+-K+ pumps to the recovery of excitability and contractility in depolarized skeletal muscle. In soleus muscle, Ba2+ (0.02 and 1 mM) was found to inhibit 42K+ efflux and 42K+ influx. Both in the absence and the presence of Ba2+ (1 mM), salbutamol and calcitonin gene-related peptide (CGRP) induced a marked decrease in intracellular Na+ and stimulation of 42K+ uptake. In soleus muscles Ba2+ (0.1 and 1.0 mM) decreased twitch and tetanic force. Subsequent stimulation of the Na+-K+ pumps by salbutamol, CGRP or repeated electrical stimulation produced a highly significant restoration of force development, which was suppressed by ouabain, but not by glibenclamide. Also, in extensor digitorum longus muscles Ba2+ (0.1 mM) produced a considerable force decline, which was partly restored by salbutamol and CGRP. The area of compound action potentials (M-waves) elicited by indirect stimulation was decreased by Ba2+ (0.1 mM). This was associated with a concomitant decrease in tetanic force and depolarization. Salbutamol, CGRP or repeated electrical stimulation all elicited marked recovery of M-wave area, force and membrane potential. All recordings showed close correlations between these three parameters. The data add further support to the concept that due to its electrogenic nature and large transport capacity, the Na+-K+ pump is a rapid and efficient mechanism for the maintenance of excitability in skeletal muscle, acting independently of Ba2+- or ATP-sensitive K+ channel function.

Figure 1. Effects of salbutamol and CGRP on tetanic force in the presence of Ba²⁺

Soleus muscles were equilibrated for 30 min in standard buffer and isometric tetanic contractions were elicited by direct stimulation using 30 Hz pulse trains of 2 s duration (1 ms pulses, 12 V). The buffer (NKR, Krebs-Ringer bicarbonate buffer) was then changed to SO₄²⁻-free buffer and after a further 15 min of equilibration, BaCl₂ (0.1 mm) was added. After 120 min, salbutamol (10⁻⁵ M) or rat CGRP (10⁻⁷ M) were added. Each point represents the mean of observations on 3–5 muscles with bars indicating s.e.m.

Figure 2. Effects of CGRP on tetanic force, M-wave area and V_m in the presence of Ba²⁺

Upper panel: muscles were stimulated tetanically (30 Hz, 1.5 s) via the nerve at the time points indicated. Each point represents the mean of observations on 4 muscles with bars indicating ± s.e.m. M-wave area, ○. Tetanic force, •. Lower panel: recordings of membrane potential were taken at the time points indicated. Each point represents the mean observation on 4 muscles (total of 40 fibres) with bars indicating ± s.e.m. (n = 4).

Figure 3. Relationships between membrane potential, M-wave area and tetanic force

Corresponding data points for simultaneous recordings of M-wave area, tetanic force and V_m. Data points were collected from 11 individual muscles in 3 groups subjected to experiments with a time course similar to that shown in Fig. 2. The data points included correspond to the times 0, 60, 150 and 180 min in Fig. 2. After incubation in SO₄²⁻-free buffer containing 0.1 mm Ba²⁺ muscles were given either CGRP (10⁻⁷ M) (as in Fig. 2) or salbutamol (10⁻⁵ M), or were stimulated at 1 min intervals to induce recovery (n = 3–4 in each group). A regression plane was fitted to the data points in the cube; the points situated above the plane are represented by open circles, and those below the plane by shaded circles.