Novel role of insulin in the regulation of glucose excretion by mourning doves (Zenaida macroura)

In mammals, insulin primarily lowers plasma glucose (PGlu) by increasing its uptake into tissues. Studies have also shown that insulin lowers PGlu in mammals by modulating glomerular filtration rate (GFR). Birds have naturally high PGlu and, although insulin administration significantly decreases glucose concentrations, birds are resistant to insulin-mediated glucose uptake into tissues. Since prior work has not examined the effects of insulin on GFR in birds, the purpose of the present study was to assess whether insulin can augment renal glucose excretion and thereby lower PGlu. Therefore, the hypothesis of the present study was that insulin lowers PGlu in birds by augmenting GFR, as estimated by inulin clearance (CIn). Adult mourning doves (Zenaida macroura) were used as experimental animals. Doves were anesthetized and the brachial vein was cannulated for administration of [14C]-inulin and insulin and the brachial artery was cannulated for blood collections. Ureteral urine was collected via a catheter inserted into the cloaca. Ten minutes following administration of exogenous insulin (400 μg/kg body mass, i.v.) plasma glucose was significantly decreased (p = 0.0003). Twenty minutes following insulin administration, increases in GFR (p = 0.016) were observed along with decreases in urine glucose concentrations (p = 0.008), glucose excretion (p = 0.028), and the fractional excretion of glucose (p = 0.003). Urine flow rate (p = 0.051) also tended to increase after administration of insulin. These data demonstrate a significant role for insulin in modulating GFR in mourning doves, which may in part explain the lower PGlu measured following insulin administration.