Heme reduces corpus cavernosum smooth muscle contraction via the HO-CO-sGC-cGMP pathway: implications for priapism in sickle cell disease.

Recurrent ischemic priapism is a common complication in males with sickle cell disease (SCD), strongly associated with chronic intravascular hemolysis. Hemolysis elevates circulating free heme levels, which are metabolized by heme oxygenase (HO) into carbon monoxide (CO). CO activates soluble guanylate cyclase (sGC), increasing cyclic GMP (cGMP) and promoting smooth muscle relaxation. This study hypothesizes that excess extracellular heme contributes to the pathophysiology of priapism by impairing corpus cavernosum contractility through the HO-CO-sGC-cGMP pathway. The aim of this study was to investigate the effects of heme on contractile mechanisms in the mouse corpus cavernosum and assess the involvement of this signaling pathway. A total of 114 male C57BL/6 mice (3-4 months old) were used. Mice corpus cavernosum was dissected free and mounted in 7-mL organ baths containing Krebs solution. Contractions were induced by phenylephrine (1 nM-300 µM), KCl (1-300 mM), or electrical field stimulation (EFS; 2-32 Hz). Tissues were pre-incubated with heme (100 µM) or vehicle (0.1% DMSO), with or without the HO inhibitor 1 J or the sGC inhibitor ODQ (10 µM, 30 min). Additional groups were pre-treated with the heme oxygenase (HO) inhibitor 1 J (10 µM) or the sGC inhibitor ODQ (10 µM) prior to heme exposure. Pre-incubation with heme significantly reduced EFS-induced contractions across all tested frequencies (e.g., 32 Hz: control 1.20 ± 0.17 mN vs. heme 0.67 ± 0.12 mN; P = 0.02; n = 6). Similarly, phenylephrine-induced contraction was attenuated by heme, with a decrease in Emax (control 0.90 ± 0.08 mN vs. heme 0.57 ± 0.11 mN; P = 0.03; n = 7), without changes in pEC₅₀. KCl-induced contractions were also affected, with reduced potency (pEC₅₀: control 1.18 ± 0.06 vs. heme 1.90 ± 0.11; P = 0.04; n = 6), though Emax remained unchanged. The inhibitory effects of heme were abolished by 1 J or ODQ in all protocols, indicating that the HO-CO-sGC-cGMP pathway mediates these responses. In conclusion, heme impairs contractile responses of the corpus cavernosum through activation of the HO-CO-sGC-cGMP signaling cascade. These findings identify a novel mechanism potentially contributing to priapism in SCD and support further investigation of this pathway as a therapeutic target.