Possibility of inhibition of calcium-activated chloride channel rescuing erectile failures in diabetes

Ion Channels and Intracellular Calcium Signalling in Corpus Cavernosum

The corpus cavernosum smooth muscle is important for both erection of the penis and for maintaining penile flaccidity. Most of the time, the smooth muscle cells are in a contracted state, which limits filling of the corpus sinuses with blood. Occasionally, however, they relax in a co-ordinated manner, allowing filling to occur. This results in an erection. When contractions of the corpus cavernosum are measured, it can be deduced that the muscle cells work together in a syncytium, for not only do they spontaneously contract in a co-ordinated manner, but they also synchronously relax. It is challenging to understand how they achieve this.In this review we will attempt to explain the activity of the corpus cavernosum, firstly by summarising current knowledge regarding the role of ion channels and how they influence tone, and secondly by presenting data on the intracellular Ca²⁺ signals that interact with the ion channels. We propose that spontaneous Ca²⁺ waves act as a primary event, driving transient depolarisation by activating Ca²⁺-activated Cl^- channels. Depolarisation then facilitates Ca²⁺ influx via L-type voltage-dependent Ca²⁺ channels. We propose that the spontaneous Ca²⁺ oscillations depend on Ca²⁺ release from both ryanodine- and inositol trisphosphate (IP₃)-sensitive stores and that modulation by signalling molecules is achieved mainly by interactions with the IP₃-sensitive mechanism. This pacemaker mechanism is inhibited by nitric oxide (acting through cyclic GMP) and enhanced by noradrenaline. By understanding these mechanisms better, it might be possible to design new treatments for erectile dysfunction.

The role of Ca2+-activated Cl- current in tone generation in the rabbit corpus cavernosum

Rabbit corpus cavernosum smooth muscle (RCCSM) cells express ion channels that produce Ca²⁺-activated Cl^- (I_ClCa) current, but low sensitivity to conventional antagonists has made its role in tone generation difficult to evaluate. We have reexamined this question using two new generation I_ClCa blockers, T16A_inh-A01 and CaCC_inh-A01. Isolated RCCSM cells were studied using the perforated patch method. Current-voltage protocols revealed that both L-type Ca²⁺ current and I_ClCa T16A_inh-A01 and CaCC_inh-A01 (10 μM) reduced I_ClCa by ~85%, while 30 μM abolished it. L-type Ca²⁺ current was unaffected by 10 μM CaCC_inh-A01 but was reduced by 50% at 30 μM CaCC_inh-A01, 46% at 10 μM T16A_inh-A01, and 78% at 30 μM T16A_inh-A01. Both drugs reduced spontaneous isometric tension in RCCSM strips, by 60-70% at 10 μM and >90% at 30 μM. Phenylephrine (PE)-enhanced tension was also reduced (ED₅₀ = 3 μM, CaCC_inh-A01; 14 μM, T16A_inh-A01). CaCC_inh-A01 at 10 μM had little effect on 60 mM KCl contractures, though they were reduced by 30 μM CaCC_inh-A01 and T16A_inh-A01 (10 μM and 30 μM) consistent with their effects on L-type Ca²⁺ current. Both drugs also reversed the stimulatory effect of PE on intracellular Ca²⁺ waves, studied with laser scanning confocal microscopy in isolated RCCSM cells. In conclusion, although both drugs were effective blockers of I_ClCa, the effect of T16A_inh-A01 on L-type Ca²⁺ current precludes its use for evaluating the role of I_ClCa in tone generation. However, 10 μM CaCC_inh-A01 selectively blocked I_ClCa versus L-type Ca²⁺ current and reduced spontaneous and PE-induced tone, suggesting that I_ClCa is important in maintaining penile detumescence.