Recently, we have demonstrated that interstitial cells isolated from the rabbit urethra fire spontaneous transient inward currents (Sergeant et al. 2000). These cells may play an important role in neurotransmission as they do in the GI tract (Ward & Sanders, 200l). The purpose of the present study was to examine the mechanism of action of noradrenaline (NA) on these cells.

Rabbits were killed with pentobarbitone (I.V.) and their urethras removed. Interstitial cells were isolated by enzymatic dispersal and studied with the amphotericin B perforated-patch technique (Cs⁺ pipettes). Cells were held at -60 mV and exposed to NA (10 µM) for 10 s periods at 75 s intervals. NA evoked either large single inward currents or a series of oscillatory inward currents that were blocked by phentolamine (1 µM, n = 6) or prazosin (1 µM, n = 5), suggesting that its effects were mediated through α1 adrenoceptors. The reversal potential of the above currents was close to 0 mV (E_Cl = 0, n = 6), suggesting that they were calcium-activated chloride currents. This idea was tested further by examining the effects of the chloride channel blockers, niflumic acid (30 µM) and anthracene 9 carboxylic acid (A-9-C, 1 mM) on the NA response. Niflumic acid decreased the current from -813 ± 185 to -140 ± 26 pA (mean ± S.E.M., n = 6, P < 0.05, paired t test), whereas A-9-C reduced it from -1253 ± 510 to -399 ± 89 pA (n = 6, P < 0.05). The current was significantly depressed (-604 ± 149 to -116 ± 63 pA, n = 6, P < 0.05) by 10 µM CPA, suggesting that NA released calcium from intracellular stores.

We next tested the effects of the IP₃ blocker 2-aminoethoxydiphenylborate (2APB, Maruyama et al. 1997) and the PLC inhibitor 2-nitro-4-carboxyphenyl-n,n-diphenylcarbamate (NCDC, 100 µM) on the NA-evoked currents. In three experiments, the response to NA was abolished after application of NCDC. Similarly, inhibition of IP₃ with 2APB (10 µM) reduced the NA-evoked currents from -923 ± 195 to -457 ± 154 pA (n = 6, P < 0.05), whereas in the presence of 100 µM 2APB the amplitude of the current was reduced further to only -120 ± 40 pA (n = 10, P < 0.05). These results suggest that the stimulation of α₁ adrenoceptors releases calcium from an IP₃-sensitive store, which in turn activates the chloride current in freshly dispersed interstitial cells of the rabbit urethra.This work was supported by The Wellcome Trust.

Maruyama, T., Kanaji, T., Nakade, S., Kanno, T. & Mikoshiba, K. (1997). J. Biochem. 122, 498-505.

Sergeant, G.P., Hollywood, M.A., McCloskey, K.D., Thornbury, K.D. & McHale, N.G. (2000). J. Physiol. 526, 359-366. abstract

Ward, S.M. & Sanders, K.M. (2001). Anat Rec. 262, 125-135.