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

The presentstudy used real-time confocal microscopy to examine the effects of the₂-adrenoceptor agonistsalbutamol on regulation of intracellularCa²⁺ concentration(Ca²⁺]_i)in myotubes derived from neonatal mouse limb muscles.Immunocytochemical staining for ryanodine receptors and skeletal musclemyosin confirmed the presence of sarcomeres. The myotubes displayedboth spontaneous and ACh-induced rapid (<2-ms rise time)Ca²⁺]_itransients. TheCa²⁺]_itransients were frequency modulated by both low and high concentrations of salbutamol. Exposure to -bungarotoxin and tetrodotoxin inhibited ACh-inducedCa²⁺]_itransients and the response to low concentrations of salbutamol but notthe response to higher concentrations. Preexposure to caffeineinhibited the subsequentCa²⁺]_iresponse to lower concentrations of salbutamol and significantly blunted the response to higher concentrations. Preexposure to salbutamol diminished theCa²⁺]_iresponse to caffeine. Inhibition of dihydropyridine-sensitive Ca²⁺ channels with nifedipine orPN-200-110 did not preventCa²⁺]_ielevations induced by higher concentrations of salbutamol. The effectsof salbutamol were mimicked by the membrane-permeant analog dibutyryladenosine 3',5'-cyclic monophosphate. Thesedata indicate that salbutamol effects in skeletal muscle predominantly involve enhanced sarcoplasmic reticulumCa²⁺ release.