P2y and P2x purinoceptors are respectively implicated in endothelium- dependent relaxation and endothelium independent contraction in human corpus cavernosum

Evidence of an excitatory purinergic innervation in mouse corpus cavernosum smooth muscle

BACKGROUND

Evidence suggests that the corpus cavernosum smooth muscle (CCSM) cells of several species, including humans, express purinergic P2X receptors, but it is not known if the corpus cavernosum has an excitatory purinergic innervation.

AIM

In this study we aimed to determine if the mouse CCSM has a functional purinergic innervation.

METHODS

Mouse CCSM myocytes were enzymatically isolated and studied using the perforated patch configuration of the patch clamp technique. Isometric tension was measured in whole cavernosum tissue subjected to electrical field stimulation (EFS) to evoke nerve-mediated responses.

OUTCOMES

The mouse CCSM myocytes expressed P2X1 receptors, and adenosine triphosphate (ATP) evoked inward currents in these cells. In addition, P2X1-mediated contractions were recorded in whole tissue in response to EFS.

RESULTS

In cells held under a voltage clamp at -60 mV, ATP (1 μm) evoked large inward currents (mean approximately 900 pA). This current rapidly declined but was repeatable at 8-minute intervals. α,β-methylene ATP (10 μM), an agonist of P2X1 and P2X3 receptors, caused a similar current that also rapidly declined. Desensitization to α,β-methylene ATP negated the effect of ATP, but the ATP effect was restored 8 minutes after washout of α,β-methylene ATP. The effect of ATP was reversibly blocked by NF449 (1 μm), a selective antagonist of P2X1 receptors. In isometric tension experiments electrical field stimulation (EFS) at 0.5-8 Hz evoked frequency-dependent contractions in the presence of l-nitro arginine (l-NO-Arg) (100 μm). When phentolamine (3 μm) and atropine (1 μm) were applied, there remained a nonadrenergic, noncholinergic component of the response to EFS, consisting mainly of a transient contraction. This was significantly reduced by NF449 (1 μm). Finally, in immunocytochemistry experiments, isolated CCSM myocytes stained positively when exposed to an antibody raised against P2X1 receptors.

CLINICAL IMPLICATIONS

Previous studies have shown that P2X1 receptors in CCSM are upregulated in diabetes. These findings, taken together with the functional evidence presented here, indicate that P2X1 receptors may provide an alternative therapeutic target for treatment of erectile dysfunction in patients with diabetes, which is known to be relatively resistant to treatment with phosphodiesterase 5 inhibitors.

STRENGTHS AND LIMITATIONS

Strengths of this study are the use of a combination of functional experiments (patch clamp) and immunocytochemical analyses to show expression of P2X1 receptors on CCSM myocytes while also performing functional experiments to show that stimulation these receptors results in contraction of CCSM. A limitation of this study was the use of animal rather than human tissue.

CONCLUSION

This investigation provides evidence that mouse corpus cavernosum smooth muscle cells express P2X1 receptors and that these receptors are involved in mediating part of the contractile response to nerve stimulation evoked by EFS.

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.

ATP evokes inward currents in corpus cavernosum myocytes

INTRODUCTION

Although adenosine triphosphate (ATP) has often been reported to relax the corpus cavernosum, this may be mediated by indirect effects, such as release of nitric oxide from the endothelium. Recent data suggest that P2X(1) receptors may be up-regulated in diabetes, and these exert an anti-erectile effect by causing the corpus cavernosum smooth muscle cells (CCSMCs) to contract. However, to date, there is no functional evidence that ATP can directly stimulate CCSMC.

AIMS

This study aims to (i) to directly examine the effect of ATP on membrane currents in freshly isolated CCSMC, where influences of endothelium and other cells are absent; and (ii) to determine the receptor subtypes, ionic currents, and Ca(2+) signals stimulated by ATP.

METHODS

CCSMCs were enzymatically dispersed from male New Zealand White rabbits for patch clamp recording and measurement of intracellular Ca(2+) in fluo-4-loaded cells using spinning disk confocal microscopy.

MAIN OUTCOME MEASURES

Patch clamp recordings were made of ATP-evoked membrane currents and spontaneous membrane currents. Spinning disk confocal imaging of intracellular Ca(2+) was performed, and the response to ATP was recorded.

RESULTS

ATP evoked repeatable inward currents in CCSMC (1st application: -675 ± 101 pA; 2nd application: -694 ± 120 pA, N = 9, P = 0.77). ATP-induced currents were reduced by suramin from -380 ± 121 to -124 ± 37 pA (N = 8, P < 0.05), by α,β-methylene ATP from -755 ± 235 to 139 ± 49 pA (N = 5, P < 0.05), and by NF449 from -419 ± to -51 ± 13 pA (N = 6, P < 0.05). In contrast, MRS2500, a P2Y1(1,12,13) antagonist, had no effect on ATP responses (control: -838 ± 139 pA; in MRS2500: -822 ± 184 pA, N = 13, P = 0.84) but blocked inward currents evoked by 2-MeSATP, a P2Y1,12,13 agonist (control: -623 ± 166 pA; in MRS2500: -56 ± 25 pA, N = 6, P < 0.05). The ATP-evoked inward current was unaffected by changing the transmembrane Cl(-) gradient but reversed in direction when extracellular Na(+) was reduced, indicating that it was a cation current.

CONCLUSIONS

ATP directly stimulates CCSMC by evoking a P2X-mediated cation current.

Effect of hypothyroidism on the purinergic responses of corpus cavernosal smooth muscle in rabbits

AIMS

Several studies have reported evidence of hormonal abnormalities in 25-35% of impotent men. Hypothyroidism has been reported to occur in 6% of impotent men. In the present study, we examined purinergic relaxation responses in hypothyroidism in an experimental rabbit model and compared them with controls to evaluate the possible involvement of the purinergic pathway.

MATERIALS AND METHODS

The study comprised 20 male New Zealand white rabbits. The rabbits were divided into two equal groups. We tested the effects of ATP, alpha beta ATP, and adenosine precontracted with phenylephrine on the isolated corpus cavernosum preparations from control and hypothyroid rabbits. We also evaluated the effects of ATP, alpha beta ATP, and adenosine on the cGMP levels in the isolated corpus cavernosum preparations from control and hypothyroid rabbits.

RESULTS

T3, T4, and testosterone levels were significantly lower in hypothyroid rabbits. ATP, alpha beta ATP, carbachol, and electrical field stimulation (EFS)-induced frequency-dependent relaxation responses in the isolated rabbit corpus cavernosum strips precontracted with phenylephrine reduced significantly (P<0.05). Adenosine-induced relaxation responses did not change significantly in hypothyroid rabbits.

CONCLUSION

Reduction of relaxation response in hypothyroid rabbits corpus cavernosum can depend on a decreased release of nitric oxide (NO) from nitrergic nerves and endothelium.

An Inhibitor of the F1 subunit of ATP synthase (IF1) modulates the activity of angiostatin on the endothelial cell surface

Angiostatin binds to endothelial cell (EC) surface F(1)-F(0) ATP synthase, leading to inhibition of EC migration and proliferation during tumor angiogenesis. This has led to a search for angiostatin mimetics specific for this enzyme. A naturally occurring protein that binds to the F1 subunit of ATP synthase and blocks ATP hydrolysis in mitochondria is inhibitor of F1 (IF1). The present study explores the effect of IF1 on cell surface ATP synthase. IF1 protein bound to purified F(1) ATP synthase and inhibited F(1)-dependent ATP hydrolysis consistent with its reported activity in studies of mitochondria. Although exogenous IF1 did not inhibit ATP production on the surface of EC, it did conserve ATP on the cell surface, particularly at low extracellular pH. IF1 inhibited ATP hydrolysis but not ATP synthesis, in contrast to angiostatin, which inhibited both. In cell-based assays used to model angiogenesis in vitro, IF1 did not inhibit EC differentiation to form tubes and only slightly inhibited cell proliferation compared with angiostatin. From these data, we conclude that inhibition of ATP synthesis is necessary for an anti-angiogenic outcome in cell-based assays. We propose that IF1 is not an angiostatin mimetic, but it can serve a protective role for EC in the tumor microenvironment. This protection may be overridden in a concentration-dependent manner by angiostatin. In support of this hypothesis, we demonstrate that angiostatin blocks IF1 binding to ATP synthase and abolishes its ability to conserve ATP. These data suggest that there is a relationship between the binding sites of IF1 and angiostatin on ATP synthase and that IF1 could be employed to modulate angiogenesis.

Effect of chronic renal failure on the purinergic responses of corpus cavernosal smooth muscle in rabbits

OBJECTIVE

To examine purinergic relaxation responses in chronic renal failure (CRF) in an experimental rabbit model, and thus evaluate the possible involvement of the purinergic system in erectile dysfunction with CRF.

MATERIALS AND METHODS

The relaxant effects of ATP were measured in strips of corpus cavernosum smooth muscle taken from control and CRF rabbits. CRF was induced in New Zealand white rabbits as previously described. Penises were excised from CRF rabbits 4 weeks after inducing uraemia. In an organ bath the strips from controls and CRF rabbit corpus cavernosum were pre-contracted with phenylephrine and increasing doses of adenosine and ATP added.

RESULTS

In the pre-contracted rabbit cavernosal tissue the relaxations induced by adenosine and ATP were unchanged in CRF.

CONCLUSION

The lack of any relaxant effect of adenosine or ATP on the relaxation of cavernosal smooth muscle in rabbits with CRF might be because the relaxant effects of these agents are endothelium-independent and the endothelial purinergic receptor density was unchanged in CRF.