Effects of phenylephrine on spontaneous activity and L-type Ca2+ current in isolated corpus cavernosum myocytes.

Introduction: Norepinephrine is important in maintaining detumescent tone in the corpus cavernosum, although the mechanism is incompletely understood. As α-adrenoceptor-induced tone is antagonized by L-type Ca(2+) channel blockers, it is usually assumed that direct modulation of this current is involved. However, the effects of α-adrenoceptor agonists have never been directly examined on L-type current in corpus cavernosum myocytes (CCSMC), leaving open other possibilities. In particular, CCSMC are now known to develop spontaneous tone via a pacemaker mechanism involving spontaneous Ca(2+) waves that activate Cl(-) currents, causing depolarization and voltage-dependent activation of L-type channels. We hypothesized that phenylephrine modulates tone via this system, rather than by directly activating L-type channels.

Aims: Examine in freshly isolated CCSMC the effect of phenylephrine on: (i) spontaneous Cl(-) currents and depolarizations; (ii) cytosolic Ca(2+) waves; and (iii) L-type current.

Methods: CCSMC were enzymatically dispersed from male New Zealand White rabbits for patch clamp recording and real time Ca(2+) imaging.

Main Outcome Measures: Spontaneous Cl(-) currents, spontaneous depolarizations, cytosolic Ca(2+) and L-type current.

Results: Phenylephrine enhanced the amplitude and frequency of spontaneous Cl(-) currents, increased the duration and frequency of spontaneous depolarizations and increased the frequency of spontaneous Ca(2+) waves. These effects were blocked by 2-aminoethoxy diphenylborate (2-APB), suggesting that they were mediated by IP(3) -induced Ca(2+) release from intracellular stores. In contrast, 2-APB had no effect on Ca(2+) transients evoked by releasing stored Ca(2+) with caffeine, suggesting that it had little effect on store Ca(2+) content. Phenylephrine depressed L-type current by around 30%. This effect was removed by blocking with 2-APB. Notably, phenylephrine failed to enhance the current, even in the presence of 2-APB. Furthermore, the phorbol ester, phorbol 12-myristate 13-acetate, had no effect on L-type current.

Conclusion: Phenylephrine effects on the corpus cavernosum are mediated by modulation of the spontaneous pacemaker mechanism, rather than by direct stimulation of L-type channels.