The effect of diazepam on potassium contractures, contraction threshold, and resting tension in rat skeletal muscles

The effects of diazepam on potassium contractures, contraction threshold, and resting tension have been examined in rat soleus muscle fibres. Two actions of the drug were defined that could not be attributed to changes in the resting membrane potential or depolarization in high potassium solutions. The major effect was an increase in the amplitude of submaximal tension during either twitches or potassium contractures and an increase in resting tension. At 400 μM diazepam, there was (a) a fourfold increase in 40 mM potassium contracture tension, (b) a negative shift of 8 mV in the membrane potential for half maximum tension estimated from the best fit of a Boltzmann-type equation to average potassium contracture data, (c) a negative shift of 8 mV in the threshold for contraction measured under voltage clamp conditions, and (d) a contracture of variable amplitude to a level that was occasionally equivalent to maximum tetanic tension. These potentiating actions of diazepam depended on drag concentration within the range of 100–800 μM. In contrast, the second effect of diazepam, depression of maximum tension by 10–15%, was independent of drug concentration between 100 and 400 μM. The results support the idea that diazepam produces an increase in resting myoplasmic calcium concentrations.

Calcium influx in contracting and paralyzed frog twitch muscle fibers.

Calcium uptake produced by a potassium contracture in isolated frog twitch fibers was 6.7 +/- 0.8 pmol in 0.7 cm of fiber (mean +/- SEM, 21 observations) in the presence of 30 microM D600. When potassium was applied to fibers paralyzed by the combination of 30 microM D600, cold, and a prior contracture, the calcium uptake fell to 3.0 +/- 0.7 pmol (11): the fibers were soaked in 45Ca in sodium Ringer for 3 min before 45Ca, in a potassium solution, was added for 2 min; each estimate of uptake was corrected for 5 min of resting influx, measured from the same fiber (average = 2.3 +/- 0.3 pmol). The calcium influx into paralyzed fibers is unrelated to contraction. This voltage-sensitive, slowly inactivating influx, which can be blocked by 4 mM nickel, has properties similar to the calcium current described by several laboratories. The paired difference in calcium uptake between contracting and paralyzed fibers, 2.9 +/- 0.8 pmol (16), is a component of influx related to contraction. Its size varies with contracture size and it occurs after tension production: 45Ca applied immediately after contracture is taken up in essentially the same amounts as 45Ca added before contraction. This delayed uptake is probably a "reflux" refilling a binding site on the cytoplasmic side of the T membrane, which had been emptied during the prior contracture, perhaps to initiate it. We detect no component of calcium uptake related to excitation-contraction coupling occurring before or during a contracture.