Effects of extracellular calcium concentration and dihydropyridines on contraction in mammalian skeletal muscle.

1. Twitches, tetanic contractions and potassium contractures were recorded isometrically from small bundles of rat soleus muscle fibres. 2. Solutions with reduced calcium concentrations (low-calcium solutions), whether buffered with EGTA (85 and 3 microM-Ca2+) or not (15 microM-Ca2+), caused an initial potentiation of contraction followed by depression. 3. The decay of potassium contractures (200 mM-potassium) was more rapid than normal in low-calcium solutions. 4. Recovery from the inactivation produced by a 200 mM-potassium contracture was slowed in low-calcium solutions but full recovery was seen within 10-15 min after return to a solution containing 2.5 mM-Ca2+. 5. Nifedipine (50 microM) in solutions containing 2.5 mM-Ca2+ potentiated contraction whereas, in low-calcium solutions, contraction was depressed and the depression was more pronounced the lower the Ca2+ concentration. 6. As with low-calcium solutions, potassium contractures decayed more rapidly in solutions containing nifedipine. Nifedipine slowed still further the rate of recovery from inactivation in low-calcium solutions. 7. (-) Bay K 8644 (50 microM) depressed contraction, increased the rate of decay of potassium contractures and slowed recovery from inactivation, like nifedipine. The racemate of Bay K 8644 was less effective. 8. In explanation of these and other observations, it is proposed that there is a dihydropyridine-binding molecule in the walls of the transverse tubular system that normally exists predominantly in a 'precursor' form at the resting membrane potential and is converted by membrane depolarization to an 'activator' form essential for excitation-contraction coupling. Conversion of the precursor to activator involves both conformational change and dissociation of calcium. Prolonged depolarization converts activator to an inactivated form by inducing further conformational change and dissociation of calcium. Recovery from inactivation requires reverse conformational changes and rebinding of calcium. The dihydropyridines affect contraction by reducing the affinity of the molecule for calcium.