Improvement of erectile function in diabetic rats by insulin: possible role of the insulin-like growth factor system

Erectile dysfunction is commonly experienced in men with diabetes mellitus. We report that the intracavernous pressure (ICP) rise in diabetic rats was 55% of the control and returned to normal following insulin (I) or insulin plus free oxygen scavenger (I + S) treatment. Insulin-like growth factor (IGF) binding protein (IGFBP) -3, -4, and -5 messenger RNA (mRNA) levels in the major pelvic ganglia (MPG) of diabetic rats were elevated by 2-fold, 2.6-fold, and 2.5-fold, respectively. Both I and I + S returned IGFBP-4 and 5 mRNA levels to normal, whereas IGFBP-3 gene expression was severely inhibited. IGFBP-2 gene expression was greatly inhibited by diabetes and was unresponsive to treatment. In the penis of diabetic rats, IGFBP-2 and -4 mRNA levels were low, whereas IGFBP-3 mRNA levels were elevated 10-fold. These effects were reversed by I and I + S. I and I + S also corrected the IGFBP-3 expression pattern. IGF-I gene expression in the penis and MPG was not significantly increased (P < 0.05) by diabetes and returned to normal levels following I or I + S treatment. Because IGFs are potent regulatory factors in vascular tone, this newly described activity of insulin may play an important role in the improvement of erectile function seen clinically and in animal models.