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 reticulum (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 (1 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 Ca²⁺ 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 Ca²⁺ sparks by ryanodine (50 μM) or SR Ca²⁺-ATPase (SERCA) cyclopiazonic acid (CPA, 20 μM) or BK_Ca channels by iberiotoxin (200 nM) or TEA (1 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 Ca²⁺ sparks/STOCs coupling mechanism.