Impairment of corpus cavernosal smooth muscle relaxation by glycosylated human haemoglobin

To examine the effect of HbA1c, an isoform of glycosylated haemoglobin (GHb, a product of non-enzymatic reactions between elevated blood glucose and haemoglobin), on nitric oxide-mediated corpus cavernosal smooth muscle relaxation, and to categorize the mechanisms involved.

Corpus cavernosal tissue from Wistar rats (300-350 g body weight) was prepared for the measurement of isometric tension. After equilibration in Krebs solution gassed with 95% O2/5% CO2 at 37 degrees C for 90 min, optimal resting tension was applied. Tissue was precontracted with 1 micromol/L noradrenaline (NAd) and either relaxed with incremental doses of acetylcholine (ACh) or sodium nitroprusside (SNP). After washout, strips were again precontracted with NAd and then incubated with pyrogallol (100 micromol/L), 100 microL of haemoglobin or 100 microL of GHb in the presence of either L-arginine (100 micromol/L), indomethacin (10 micromol/L), allopurinol (100 micromol/L), deferoxamine (100 micromol/L), catalase (600 IU/mL), or superoxide dismutase (SOD) (120 IU/mL) before ACh- or SNP-induced relaxation responses were repeated.

Haemoglobin and GHb significantly impaired the relaxation of rat corpus cavernosum to ACh in a dose-dependent manner. L-arginine reversed the impairment caused by Hb, but not GHb. A donor of superoxide anions, pyrogallol, mimicked this impairment to ACh when added to control strips. Catalase, deferoxamine, indomethacin and allopurinol had no significant effect on the impaired relaxation response to ACh, whilst L-arginine partially reversed it. SOD completely reversed the GHb-induced impaired relaxation; GHb did not alter the relaxation response to SNP.

GHb significantly impairs endothelial NO-mediated corpus cavernosal relaxation in the rat, in vitro. This effect is caused partly by the generation of superoxide anions and the extracellular inactivation of NO.