Evidence that a Ca2+ sparks/STOCs coupling mechanism is responsible for the inhibitory effect of caffeine on electro-mechanical coupling in guinea pig ureteric smooth muscle

Recent studies have highlighted the role of the sarcoplasmic reticulurn (SR) in controlling excitability, Ca2+, signalling and contractility in smooth muscle. Caffeine, an agonist of ryanodine receptors (RyRs) on the SR has been previously shown to effect Ca2+, signalling but its effects on excitability and contractility are not so clear. We have studied the effects of low concentration of caffeine (I mM) on Ca2+ signalling, action potential and contractility of guinea pig ureteric smooth muscle. Caffeine produced reversible inhibition of the action potentials, Ca2+, transients and phasic contractions evoked by electrical stimulation. It had no effect on the inward Ca2+ current or Ca 21 transient but increased the amplitude and the frequency of spontaneous transient outward currents (STOCs) in voltage clamped ureteric myocytes, suggesting Ca2+ activated K+ channels (BK) are affected by it. In isolated cells and cells in situ caffeine produced an increase in the frequency and the amplitude of Ca2+ sparks as well the number of spark discharging sites per cell. Inhibition of Ca 2, sparks by ryanodine (50 mu M) or SR Ca2+ ATPase (SERCA) cyclopiazonic acid (CPA, 20 mu M) or BKCa channels by iberiotoxin (200 nM) or TEA (I mM), fully reversed the inhibitory effect of caffeine on Ca2+ transients and force evoked by electrical field stimulation (EFS). These data suggest that the inhibitory effect of caffeine on the action potential, Ca2+ transients and force in ureteric smooth muscle is caused by activation of Ca 2, sparks/STOCs coupling mechanism. (c) 2007 Elsevier Ltd. All rights reserved.