Differential sensitivity to perchlorate and caffeine of tetracaine-resistant Ca²⁺ release in frog skeletal muscle

Abstract

In voltage clamped frog skeletal muscle fibres 0.2 mM tetracaine strongly suppresses Ca²⁺ release. After this treatment Ca²⁺ release flux lacks its characteristic initial peak and the remaining steady component is strongly reduced when compared with the control condition. We studied the effect of two agonists of Ca²⁺ release on these tetracaine treated fibres. 8 mM ClO ⁻₄ added after tetracaine potentiated release flux from 0.11 ± 0.03 mM s⁻¹ to 0.34 ± 0.07 mM s⁻¹ (n = 6) although without recovery of the peak at any test voltage. The voltage dependence of the increased release was shifted towards more negative potentials (approximately −10 mV). The effects of ClO ⁻₄ on charge movement under these conditions showed the previously described characteristic changes consisting in a left shift of its voltage dependence (approximately −9 mV) together with a slower kinetics, both at the ON and OFF transients. Caffeine at 0.5 mM in the presence of the same concentration of tetracaine failed to potentiate release flux independently of the test voltage applied. When the cut ends of the fibre were exposed to a 10 mM BAPTA intracellular solution, in the absence of tetracaine, the peak was progressively abolished. Under these conditions caffeine potentiated release restoring the peak (from 0.63 ± 0.12 mM s⁻¹ to 1.82 ± 0.23 mM s⁻¹) with no effect on charge movement. Taken together the present results suggest that tetracaine is blocking a Ca²⁺ sensitive component of release flux. It is speculated that the suppressed release includes a component that is dependent on Ca²⁺ and mainly mediated by the activation of the β ryanodine receptors (the RyR3 equivalent isoform). These receptors are located parajunctionally in the frog and are not interacting with the dihydropyridine receptor.