Objective: To study the effects of chronic insulin administration without sugar supplementation on blood pressure and response to acute saline loading in normal rats.
Design: Comparison of blood pressure, insulin and glucose levels in 24 insulin-treated and 12 control rats on regular rat chow (not supplemented with sugar).
Methods: Sustained-release insulin implants (or sham implantation for the control rats) were administered subcutaneously. The sustained-release insulin implant size was gradually increased. Tail-cuff systolic blood pressure, insulin and glucose were measured twice a week for 8 weeks, after which intra-arterial blood pressure was recorded under resting conditions and 2 h after saline loading in seven insulin-treated and seven control rats.
Results: Insulin-treated rats had a 1.2- to twofold increase in insulin without hypoglycaemia, a small but significant increase in glucose levels being found at weeks 6 and 8. When the rats were killed (week 8) triglyceride and fructosamine levels were increased in the insulin-treated rats in comparison with controls. Neither tail-cuff systolic blood pressure nor resting intra-arterial blood pressure differed between the two groups. However, acute saline loading resulted in significantly higher blood pressure in the insulin-treated rats, without altering renal Na+ excretion.
Conclusions: It is possible to produce mild hyperinsulinaemia without hypoglycaemia by gradually increasing subcutaneous sustained-release insulin administration without sugar supplementation. Such hyperinsulinaemia is associated with significantly higher glucose, fructosamine and triglyceride levels, and normal tail-cuff and resting intra-arterial blood pressure. Insulin may induce intolerance to acute volume loading that is not associated with Na+ retention.