Effect of insulin on in vivo glucose utilization in individual tissues of anesthetized lactating rats.

Glucose utilization rate has been measured in skeletal muscles, white adipose tissue, and mammary gland of anesthetized nonlactating and lactating rats. During lactation, basal glucose utilization is decreased by 40% in periovarian white adipose tissue and by 65% in epitrochlearis and extensor digitorum longus but not in soleus muscle. This may be related to the lower blood glucose and plasma insulin concentrations observed during lactation. Basal glucose utilization rate in the mammary gland was, respectively, 18 +/- 2 and 350 +/- 50 micrograms/min in nonlactating and lactating rats. During the euglycemic hyperinsulinemic clamp, a physiological increment in plasma insulin concentration (231 +/- 18 in lactating vs. 306 +/- 24 microU/ml in nonlactating rats) induces a similar increase in glucose utilization rate in skeletal muscles (except soleus) and white adipose tissue in the two groups of rats. Furthermore this low increase in plasma insulin concentration does not alter mammary glucose utilization rate in nonlactating rats but induces the same increase (sevenfold over basal) as a maximal insulin concentration in lactating rats. These data show that the active mammary gland is the most insulin-sensitive tissue of the lactating rat that has been tested. The overall increase in insulin sensitivity and responsiveness that has been described in lactating rats can then mainly be attributed to the presence of the active mammary gland.