Heijden, Erik H.F.M. van der

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.

The effects of the β2-adrenoceptor agonist salbutamol (Slb) on isometric and isotonic contractile properties of the rat diaphragm muscle (Diamus) were examined. A loading dose of 25 μg/kg Slb was administered intracardially before Diamusexcision to ensure adequate diffusion. Studies were then performed with 0.05 μM Slb in the in vitro tissue chamber. cAMP levels were determined by radioimmunoassay. Compared with controls (Ctl), cAMP levels were elevated after Slb treatment. In Slb-treated rats, isometric twitch and maximum tetanic force were increased by ∼40 and ∼20%, respectively. Maximum shortening velocity increased by ∼15% after Slb treatment, and maximum power output increased by ∼25%. During repeated isotonic activation, the rate of fatigue was faster in the Slb-treated Diamus, but both Slb-treated and Ctl Diamusfatigued to the same maximum power output. Still, endurance time during repetitive isotonic contractions was ∼10% shorter in the Slb-treated Diamus. These results are consistent with the hypothesis that β-adrenoceptor stimulation by Slb enhances Diamuscontractility and that these effects of Slb are likely mediated, at least in part, by elevated cAMP.

Prakash, Y. S., H. F. M. van der Heijden, M. S. Kannan, and G. C. Sieck. Effects of salbutamol on intracellular calcium oscillations in porcine airway smooth muscle. J. Appl. Physiol. 82(6): 1836–1843, 1997.—Relaxation of airway smooth muscle (ASM) by β-adrenoceptor agonists involves reduction of intracellular Ca2+concentration ([Ca2+]i). In porcine ASM cells, acetylcholine induces [Ca2+]ioscillations that display frequency modulation by agonist concentration and basal [Ca2+]i. We used real-time confocal microscopy to examine the effect of salbutamol (1 nM to 1 μM), a β2-adrenoceptor agonist, on [Ca2+]ioscillations in freshly dissociated porcine ASM cells. Salbutamol decreased the frequency of [Ca2+]ioscillations in a concentration-dependent fashion, completely inhibiting the oscillations at 1 μM. These effects were mimicked by a cell-permeant analog of adenosine 3′,5′-cyclic monophosphate. The inhibitory effect of salbutamol was partially reversed by BAY K 8644. Salbutamol reduced [Ca2+]ieven when sarcoplasmic reticulum (SR) Ca2+reuptake and Ca2+influx were blocked. Lanthanum blockade of Ca2+efflux attenuated the inhibitory effect of salbutamol on [Ca2+]i. The [Ca2+]iresponse to caffeine was unaffected by salbutamol. On the basis of these results, we conclude that β2-adrenoceptor agonists have little effect on SR Ca2+release in ASM cells but reduce [Ca2+]iby inhibiting Ca2+influx through voltage-gated channels and by enhancing Ca2+efflux.