Effect of beta-adrenoceptor activation on [Ca2+](i) regulation in murine skeletal myotubes

The present study used real-time confocal microscopy to examine the effects of the beta(2)-adrenoceptor agonist salbutamol on regulation of intracellu lar Ca2+ concentration ([Ca2+](i)) in myotubes derived from neonatal mouse limb muscles. Immunocytochemical staining for ryanodine receptors and skele tal muscle myosin confirmed the presence of sarcomeres. The myotubes displa yed both spontaneous and ACh-induced rapid (<2-ms rise time) [Ca2+](i) tran sients. The [Ca2+](i) transients were frequency modulated by both low and h igh concentrations of salbutamol. Exposure to a-bungarotoxin and tetrodotox in inhibited ACh-induced [Ca2+](i) transients and the response to low conce ntrations of salbutamol but not the response to higher concentrations. Pree xposure to caffeine inhibited the subsequent [Ca2+](i) response to lower co ncentrations of salbutamol and significantly blunted the response to higher concentrations. Preexposure to salbutamol diminished the [Ca2+](i) respons e to caffeine. Inhibition of dihydropyridine-sensitive Ca2+ channels with n ifedipine or PN-200-110 did not prevent [Ca2+](i) elevations induced by hig her concentrations of salbutamol. The effects of salbutamol were mimicked b y the membrane-permeant analog dibutyryl adenosine 3',5'-cyclic monophospha te. These data indicate that salbutamol effects in skeletal muscle predomin antly involve enhanced sarcoplasmic reticulum Ca2+ release.