Myosin heavy chain isoform transitions in the developing rat cremasteric muscle correspond with testicular descent

It has been proposed that in the rat, the cremasteric muscle exerts a mechanical force upon the intra-abdominal testis, pulling the testis into the scrotum during development. Since myosin heavy chain (MHC) isoform expression correlates with mechanical properties of muscle fibers, we hypothesized that in the cremasteric muscle, the normal postnatal transitions in MHC isoform expression to adult fast isoforms corresponds temporally with testicular descent. Male Sprague-Dawley rats were euthanized on postnatal days (P) 0, 7, 14, 21, 28 and adults (300 g). The cremasteric muscle was excised. MHC isoform composition was determined by polyacrylamide gel electrophoresis under denaturing conditions. Gels were silver stained, and the relative distribution of each MHC isoform was determined by densitometric analysis. From P0 through P14, the MHC-Neo isoform predominated (>65%). In contrast, in the adult, the MHC-2X isoform predominated (∼80% of total MHC). The half-transition time to adult isoforms occurred at P18, just preceding normal testicular descent on P21. We conclude that with the transition in MHC isoform expression to adult fast MHC isoforms, mechanical performance of the cremasteric muscle improves and that this transition is coincident with testicular descent.