Characterization of norepinephrine-evoked inward currents in interstitial cells isolated from the rabbit urethra

Smooth Muscle Group, Department of Physiology, The Queen's University of Belfast, BT9 7BL Belfast, United Kingdom Freshly dispersed interstitial cells from the rabbit urethra were studied by using the perforated-patch technique. When cells were voltage clamped at 60 mV and exposed to 10 µM norepinephrine (NE) at 80-s intervals, either large single inward currents or a series of oscillatory inward currents of diminishing amplitude were evoked. These currents were blocked by either phentolamine (1 µM) or prazosin (1 µM), suggesting that the effects of NE were mediated via 1 -adrenoceptors. NE-evoked currents were depressed by the blockers of Ca 2+ -activated Cl currents, niflumic acid (10 µM), and 9-anthracenecarboxylic acid (9-AC, 1 mM). The reversal potential of the above currents changed in a predictable manner when the Cl equilibrium potential was altered, again suggesting that they were due to activation of a Cl conductance. NE-evoked currents were decreased by 10 µM cyclopiazonic acid, suggesting that they were dependent on store-released Ca 2+ . Inhibition of NE-evoked currents by the phospholipase C inhibitor 2-nitro-4-carboxyphenyl- N , N -diphenylcarbamate (100 µM) suggested that NE releases Ca 2+ via an inositol 1,4,5-trisphosphate (IP 3 )-dependent mechanism. These results support the idea that stimulation of 1 -adrenoceptors releases Ca 2+ from an IP 3 -sensitive store, which in turn activates Ca 2+ -activated Cl current in freshly dispersed interstitial cells of the rabbit urethra. This elevates slow wave frequency in these cells and may underlie the mechanism responsible for increased urethral tone during nerve stimulation. chloride; pacemaker cell; neurotransmission