Four mid-lactation Holstein dairy cows (mean milk yield on day of
experiments 26·1 kg/d) were used in a series of experiments to
establish the contribution of non-insulin-mediated glucose uptake
to total glucose uptake at basal
insulin concentrations. A secondary objective was to determine whether
somatostatin affects the action of infused insulin. In part I of the
experiment a primed continuous infusion of [6,6-2H]glucose
(45·2 μg/kg per min) was begun at time 0 and
continued for 5 h. After 3 h of [6,6-2H]glucose
infusion (basal period) a primed
continuous infusion of insulin (0·001 i.u./kg per min) was
administered for 2 h.
Coincident with the insulin infusion, normal glucose was also infused in order to
maintain the plasma glucose concentration at euglycaemia. Part II of the experiment
was the same as part I except that somatostatin was infused for 2 h
(0·333 μg/kg per
min) instead of insulin. In part III of the experiment both insulin and somatostatin
were infused for the final 2 h. Plasma insulin levels were
increased by insulin infusion
(to 0·1476 and 0·1290 i.u./l for parts I and
III respectively) and were reduced by
somatostatin infusion in part II (to 0·006 i.u./l) relative
to the basal periods (mean
0·021 i.u./l). Glucose uptake during somatostatin infusion
(2·50 mg/kg per min; part
II) was 92·0% of that observed in the respective basal period
(2·72 mg/kg per min).
Circulating insulin levels were much lower than the dose of
insulin that causes a half
maximal effect on glucose uptake (0·06–0·10 i.u./l
for ruminants); consequently
insulin-mediated glucose uptake was probably absent in part II. Secondly, glucose
uptake following insulin only infusion (4·05 mg/kg per min) was
significantly lower than that observed when insulin plus somatostatin was infused
(4·69 mg/kg per min),
indicating that somatostatin either directly or indirectly enhanced the action of
insulin on glucose uptake.