Mechanical Responses and Calcium Influx in Frog Muscles: Effects of low Temperature and Cooling

Abstract

Isotonic potassium contractures of frog sartorii are 3-5 times prolonged at 0 °C and are larger in amplitude. the Ca influx measured by Ca⁴⁵ entry, connected with activity, is 1.65 times larger during contractures at 0 °C than at 20-22° C. the increased influx into active muscles does not seem to involve a more firm binging of Ca on the surface of muscle fibres. Passive (resting) influx of Ca is diminished by about 20% at low temperature. Contractions can be provoked by cooling in muscles pretreated with Ca-free Ringer solution. These contractions may be abolished by omission of NaCl from the medium or by procaine. Submaximal responses are provoked by cooling in muscles pretreated for a few minutes by solutions containing 1 mM or more Ca and 30 mM or more K ions. Ca⁴⁵ influx into such muscles is about 50% less than the influx in similarly treated but not cooled muscles. Maximal contractures are provoked by cooling muscles pretreated with solutions containing subthreshold concentrations of caffeine. These responses are unaffected by changing the Ca and K concentrations of the medium in wide limits.