Modulation of Ca²⁺ channels, charge movement and Ca²⁺ transients by heparin in frog skeletal muscle fibres

Summary

This study is an investigation into the modulatory effects of heparin, a component of the extracellular matrix that binds to dihydropyridine receptors, on contraction and Ca²⁺ channels in frog skeletal muscle. Using tension and Ca²⁺ signal measurements in single intact skeletal muscle cells we have found that heparin (100–200 μg ml^-1) substantially potentiates twitch and tetanic tension (55% and 28%, respectively). In contrast, heparin reduces the amplitude of K⁺ contractures. Heparin most likely potentiates twitch tension by prolonging action potentials. The ionic basis of this effect was investigated in voltage-clamp experiments. Membrane currents were monitored in voltage-clamped segments of single fibres using the triple Vaseline gap technique. We found that heparin partially blocks delayed rectifier potassium channels. The depressive effects of heparin on K⁺ contractures prompted us to investigate the effects of heparin on charge movement and Ca²⁺ currents (I _Ca) under voltage-clamp. Charge movement was measured using a subtraction procedure that employed a -20 mV control pulse from a holding potential of 100 mV. Heparin depresses the total charge by 25%. We propose that the reduction in the amplitude of potassium contractures is related to a partial blockade of charge movement. Extracellular heparin shifts the I _Ca-V relation toward more negative voltages and delays the deactivation of tail currents. Double pulse experiments revealed that conditioning depolarizations speed the activation of I _Ca during test depolarizations. Heparin does not affect this process. The primary action of heparin is to accelerate the activation of I _Ca during pulses not preceded by conditioning depolarizations. Overall, the kinetic effects of heparin on I _Ca would increase the Ca²⁺ influx associated with action potentials. However, mechanical and optical experiments performed in Ca²⁺-free solutions and in the presence of Ca²⁺ channel blockers revealed that twitch and tetanic potentiation occur even in the absence of Ca²⁺-influx.