Intravascular Hemolysis Leads to Priapism Phenotype: Experimental Evidence

Introduction: Sickle cell disease (SCD) men display priapism. Berkeley and Townes SCD transgenic mice also display priapism phenotype associated with increased corpus cavernosum relaxation. Primary mechanism underlying priapism appears to be due to reduced nitric oxide bioavailability, which leads to downregulation of phosphodiesterase type 5 (PDE5) in the penis. Previous clinical studies reported that priapism is associated with increased intravascular hemolysis at SCD. Intravascular hemolysis releases components that impair nitric oxide signaling, such as hemoglobin, asymmetric dimethylarginine and arginase-1. It is well-established that nitric oxide scavenging by cell-free hemoglobin contributes to vascular dysfunction. However, no detailed study has investigated the role of intravascular hemolysis on pathophysiological alterations of erectile function. Since intravascular hemolysis can influence the biological activity of nitric oxide pathway, we hypothesized that excess concentration of plasma hemoglobin may contribute to priapism in SCD. Therefore, in the present study, we have used a murine model of intravascular hemolysis induced by phenylhydrazine (PHZ) to evaluate the effects of intravascular hemolysis on the nitric oxide-mediated corpus cavernosum relaxations.

Methods: Intravascular hemolysis was induced in C57BL/6 mice by intraperitoneal injection of PHZ. C57BL/6 mice were injected with PHZ at 50 mg/Kg and then reinjected with 50 mg/Kg 8h later. Mice were killed in isoflurane chamber at 2 days after the first injection with PHZ. Strips of corpus cavernosum were mounted in isolated organ baths, and the relaxing responses to acetylcholine (ACh; endothelium-dependent response) and sodium nitroprusside (SNP; endothelium-independent response), as well as electrical-field stimulation (EFS; nitrergic relaxation) were obtained in corpus cavernosum strips precontracted with the α1-adrenergic receptor agonist phenylephrine (10 µM).

Results: Plasma hemoglobin was significantly higher in PHZ compared to control mice (11±1 and 4±1 µM, respectively; P<0.001), which demonstrate the increase in the rate of intravascular hemolysis in PHZ-treated animals. The cumulative addition of ACh (0.001-10 µM) produced concentration-dependent corpus cavernosum relaxations in both groups, but maximal relaxations were significantly higher in PHZ (59±4 %; P<0.05, n=7) compared to control mice (45±4%). Likewise, SNP (0.01-10 µM) produced concentration-dependent corpus cavernosum relaxations, but, again, the maximal relaxations induced by this agent were significantly higher in PHZ (87±2%, P<0.01) compared to the control group (74±1%, n=7). Similarly, the nitrergic relaxations caused by EFS were also significantly higher (P<0.05) in PHZ mice compared to control mice (32 Hz: 79±2 and 66± 1%, respectively; n=5).

Conclusion: Our study shows that intravascular hemolysis leads to increased nitric oxide-mediated cavernosal relaxations, indicating that PHZ mice exhibit priapism phenotype. It is likely that hemoglobin act to inactivate nitric oxide, thus reducing the amount of bioavailable nitric oxide, which may lead to downregulation of PDE5 in the penis. One may speculate, therefore, that the reduced degradation of cGMP by PDE5 in PHZ mice may lead to the enhancement of nitric oxide-mediated cavernosal relaxations. To confirm this hypothesis, we will evaluate the expression of PDE5 and cGMP levels in PHZ mice penises.

Financial Support: FAPESP.