Gallant, Esther M.

The present study used real-time confocal microscopy to examine the effects of the β2-adrenoceptor agonist salbutamol on regulation of intracellular Ca2+concentration ([Ca2+]i) in myotubes derived from neonatal mouse limb muscles. Immunocytochemical staining for ryanodine receptors and skeletal muscle myosin confirmed the presence of sarcomeres. The myotubes displayed both spontaneous and ACh-induced rapid (<2-ms rise time) [Ca2+]itransients. The [Ca2+]itransients were frequency modulated by both low and high concentrations of salbutamol. Exposure to α-bungarotoxin and tetrodotoxin inhibited ACh-induced [Ca2+]itransients and the response to low concentrations of salbutamol but not the response to higher concentrations. Preexposure to caffeine inhibited the subsequent [Ca2+]iresponse to lower concentrations of salbutamol and significantly blunted the response to higher concentrations. Preexposure to salbutamol diminished the [Ca2+]iresponse to caffeine. Inhibition of dihydropyridine-sensitive Ca2+channels with nifedipine or PN-200-110 did not prevent [Ca2+]ielevations induced by higher concentrations of salbutamol. The effects of salbutamol were mimicked by the membrane-permeant analog dibutyryl adenosine 3′,5′-cyclic monophosphate. These data indicate that salbutamol effects in skeletal muscle predominantly involve enhanced sarcoplasmic reticulum Ca2+release.