Effect of diabetes on fast response to norepinephrine in rat aorta

The fast and slow components of the mechanical response to 1 microM norepinephrine (NE) were measured in aortic rings isolated from eight spontaneously diabetic rats, six streptozocin-induced diabetic (STZ-D) rats, six STZ-D rats treated with 2.5 U insulin/day during the 4 days before being killed, and six age- and sex-matched control rats. The total contraction to NE (i.e., the sum of fast and slow components) was similar in the four groups: spontaneously diabetic, 16.53 +/- 1.72 mN; STZ-D, 15.68 +/- 1.41 mN; insulin-treated, 16.17 +/- 2.05 mN; and control, 15.27 +/- 0.96 mN (NS). The fast component, measured graphically in a total contraction in 1.35 mM Ca, was greater in spontaneously diabetic (12.61 +/- 1.07 mN, P less than 0.05) and STZ-D (12.25 +/- 0.89 mN, P less than 0.05) rats compared with control (9.14 +/- 0.74 mN) or insulin-treated (8.58 +/- 1.23 mN) rats. The same increase of the fast component was detectable after 3 min of incubation in Ca-free medium + 2 mM EGTA (control 6.54 +/- 0.47 mN, spontaneously diabetic 9.07 +/- 0.76 mN, P less than 0.05; STZ-D 8.82 +/- 0.72 mN, P less than 0.05), and it was also abolished by insulin treatment (insulin-treated 6.29 +/- 0.36 mN). We conclude that the diabetic state increases the fast component of NE-induced contraction either in the absence or presence of Ca in the medium. This suggests that such an increase depends on a larger release of Ca from intracellular stores.