Heme Reduces the Contraction of Corpus Cavernosum Smooth Muscle through the HO-CO-sGC-cGMP Pathway: Its Implications for Priapism in Sickle Cell Disease

Introduction: Sickle cell disease (SCD) men display priapism. Clinical studies have shown a strong positive correlation between priapism and high levels of intravascular hemolysis in men with SCD. In SCD, the accumulation of plasma-free heme results from intravascular hemolysis. One of the byproducts of heme metabolization by heme oxygenase (HO) is carbon monoxide, which can promote smooth muscle relaxation via the soluble guanylate cyclase (sGC)-cyclic guanosine monophosphate (cGMP) pathway. We hypothesize that the excessive plasma heme caused by intravascular hemolysis contributes to priapism in SCD patients. The objective of this study was to evaluate the effect of the heme group on the contractile mechanisms of the corpora cavernosa smooth muscle induced by stimulation of alpha-1 adrenergic receptors, neurogenic contraction, and receptor-independent induced contraction.

Methods: Male C57BL/6 mice, aged 3-4 months, were used for this study.Corpus cavernosum (CC) strips were mounted in isolated organ baths, and the contractile responses to the α-1 adrenergic agonist phenylephrine (1 nM - 300 µM) and KCl (1 - 300 mM) in tissues pre-incubated with heme (100 µM) or vehicle (DMSO, 0.1%) for 30 minutes were obtained in CC strips, as well as noradrenergic contraction induced by electrical-field stimulation (EFS; 2-32 Hz). The contractile response induced by these agents was also evaluated in tissues pre-incubated with 1J (heme oxygenase inhibitor; 100 µM) or ODQ (sGC inhibitor; 10 µM) for 30 minutes prior to pre-incubation with heme (100 µM) or vehicle (DMSO).

Results: The maximum response (Emax) to phenylephrine was significantly (P < 0.05) reduced in the corpus cavernosum pre-incubated with heme (0.57±0.11 mN; n=7) compared to the control group (0.90±0.08 mN; n=7), with no difference observed in potency (pEC₅₀) values. The maximum response to KCl was not altered following pre-incubation with heme (100 µM) compared to the control group However, the pEC₅₀ values for KCl were significantly lower (P < 0.05) in the heme group compared to the control group (0.90 ± 0.11 and 1.18 ± 0.06, respectively; n=7). The noradrenergic contractile response induced by EFS was significantly reduced (P < 0.05) in the corpus cavernosum pre-incubated with heme, compared to the control group, across all frequencies studied. The contractile response induced by these agents was also evaluated in tissues pre-incubated with 1J (a heme oxygenase inhibitor; 10 µM) or ODQ (a guanylate cyclase inhibitor; 10 µM) for 30 minutes prior to pre-incubation with heme (100 µM) or vehicle (DMSO). There was no difference between Emax or pEC₅₀ values for phenylephrine and KCl between the control-ODQ and Heme-ODQ groups, nor between the control-1J and Heme-1J groups. Similarly, there was no difference in neurogenic contraction between the control-ODQ and Heme-ODQ groups, nor between the control-1J and Heme-1J groups.

Conclusion: Heme reduces smooth muscle contraction of corpus cavernosum in C57BL/6 mice. The pharmacological effects of heme were blocked by inhibition of HO and sGC, indicating that the heme mechanism is dependent on HO and sGC. It is likely that activation of the CO-sGC-cGMP pathway resulting from excess free heme may contribute to priapism in SCD. Our study points to heme as a novel pharmacological target for the treatment of priapism and opens the way for new therapies that seek to reduce excess heme in plasma.

Financial Support: FAPESP (2019/18886-1; 2017/08122-9).

No relevant conflicts of interest to declare.