抄録
In bovine ciliary muscle, stimulation of M3-muscarinic receptor opens two types of non-selective cation channel with different unitary conductances of 35 pS and 100 fS which serve as major pathways for Ca2+ entry during the sustained contraction. Here we studied the effects of caffeine, an activator of Ca2+ release from intracellular stores, on the intracellular Ca2+ concentration ([Ca2+]i) and NSCC currents. We also examined the existence of STIM1, a putative calcium sensor of the endoplasmic reticulum. Methods Smooth muscle cells enzymatically isolated from the ciliary body of bovine eyes obtained from a local slaughterhouse were used. The [Ca2+]i was recorded using Fluo-4 as the indicator. Results In the normal saline solution containing 2.4 mM Ca2+, superfusion of caffeine (10 mM) caused an elevation of [Ca2+]i which showed an initial peak followed by a plateau. In the absence of extracellular Ca2+, the initial peak of the caffeine-induced response was only moderately reduced whereas the plateau was abolished.In myocytes under whole-cell voltage clamp at the holding potential of -50 mV, caffeine induced an opening of 100 fS-NSCC whereas it failed to open 35 pS-NSCC. The expression of STIM1 in bovine ciliary muscle was confirmed by RT-PCR and immunofluorescence microscopy. Conclusion Further study is encouraged to examine the possibility that involvement of signals elicited by depletion of intracellular Ca2+ store in the activation of NSCCs by muscarinic stimulation. [J Physiol Sci. 2008;58 Suppl:S80]
