Relation of Muscle Receptive Substance to the Contractile Mechanism. Responses to Heat and Drugs

Abstract

This paper reports further work in an investigation of muscle-receptive substance and contractile mechanism in smooth muscle fibers, already reported briefly. 1 2 3 The results here reported seem to confirm the hypothesis that “This receptive substance is very labile to heat.” 2 They seem to prove that the receptive substance can be destroyed without “killing” the contractile mechanism. The results, therefore, suggest that the receptive substance is not, as Langley 4 postulated, a side chain of the contractile mechanism, but rather an independent structure or substance as postulated by Dixon. 5

Method. The muscles here studied were the iris muscles of fetal pigs about 5 days before birth (length, 260 mm, plus or minus 10 mm). The fetuses were obtained from the recently killed mother, quickly excised, measured, and the marked heads placed in a mixture of hog saline and saline ice. For testing, the eyes of 7 fetuses of a litter were excised, placed in small individual pyrex glass cups in pairs, and covered with a glass plate. One eye of each pair was used as a control, the other for the test. The action of a drug or of a temperature change was measured by measuring the short diameter of the pupil before and after application. This measurement was made with fine bow dividers under illumination by both transmitted and reflected light, and the value read off on a diagonal scale. This value was corrected for any simultaneous change in the control.

To determine the lability of the receptive substance to heat of 40°C, 6 of the 7 pairs of eyes were placed in a thermoregulated physiological saline bath at 40°C and a pair removed at regular intervals and cooled in a mixutre of saline and saline ice.