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

Abstract

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 α₁-adrenoceptors. NE-evoked currents were depressed by the blockers of Ca²⁺-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 Ca²⁺. Inhibition of NE-evoked currents by the phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (100 μM) suggested that NE releases Ca²⁺ via an inositol 1,4,5-trisphosphate (IP₃)-dependent mechanism. These results support the idea that stimulation of α₁-adrenoceptors releases Ca²⁺ from an IP₃-sensitive store, which in turn activates Ca²⁺-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.