A quantitative morphologic study of the capillary endothelium of striated muscle was performed in diabetic rats one hour after injection of insulin. Diabetic and nondiabetic rats injected with saline constituted the reference groups. In a randomized sample of capillary profiles, morphometric data on micropinocytotic vesicles (MPV) and on various capillary characteristics were collected.

The numeric density of MPV was higher in diabetic rats injected with insulin than in diabetic rats injected with saline (44.2 ± 8.5 versus 34.0 ± 4.5 MPV/μ2 endothelial cytoplasm (mean ± S.D.), 2p = 0.045), thereby approaching the control value of 48.0 ± 7.3, 2p = 0.51. The insulin-injected animals, furthermore, differed markedly from the diabetic reference group as regards the ratio of free-to-attached vesicles, since they showed a large increase in the numeric density of free MPV (1.40 ± 0.18 versus 1.00 ± 0.23/μ circumference, 2p = 0.015, control value 1.59 ± 0.20) and a small decrease in the density of attached MPV (0.19 ± 0.03 versus 0.23 ± 0.02/μ circumference, 2p = 0.022; controls: 0.15 ± 0.04). Both the height of the endothelial cytoplasm and the size of the MPV were similar in all groups.

These findings may be the morphologic counterpart of the clinically demonstrated decrease in plasma volume and in the intravascular pool of albumin in diabetic patients after insulin injection. The present results demonstrate that insulin has an acute effect on the endothelial cells, including a change in the kinetics of the vesicular transport pathway

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