The Effects of β-Adrenergic Stimulation on Post-Tetanic Potentiation of Concentric Force in Fast Skeletal Muscle From Wildtype and skMLCK Devoid Mice.

Abstract

Stimulation induced myosin RLC phosphorylation, catalyzed by the skeletal myosin light chain kinase (skMLCK) is known as the primary mechanism for twitch force potentiation (PTP). We assessed concentric PTP in the absence and presence of epinephrine (1 μM). To this end, extensor digitorum longus (EDL) muscles from wildtype (WT) and skMLCK-/- (KO) mice were incubated for 30-minutes in normal (CON) or epinephrine containing (EPI) Tyrode’s solution (in vitro, 25°C); undergoing a series of twitches before and after a standard conditioning stimulus (CS; 4x100 Hz) to determine PTP (post-CS/pre-CS). Epinephrine initially enhanced PTP compared to the WT and KO control values, respectively; peaking at 19.3 ± 1.6% and 15.7 ± 10.2% by the 8-minute mark (P < 0.0001) without altering myosin phosphorylation (P = 0.503). WT muscles were significantly elevated above KO values at all time points (P < 0.05); with the exception of the final 8-minute value (P = 0.172). However, both genotypes showed similar responses to epinephrine (r2 = 0.895), demonstrating that epinephrine may primarily act on a pathway independent of myosin phosphorylation.