Intravenously infused adenosine increases the blood flow to brown adipose tissue in rats

The effects of intravenously infused adenosine (ADO) and its analogues, N6-R(-)-phenylisopropyladenosine (R-PIA), 5'-N-ethylcarboxamide adenosine (NECA), on the blood flow to interscapular brown adipose tissue were studied in urethane-anesthetized rats. ADO and NECA, but not R-PIA, caused a dose-dependent increase in interscapular brown adipose tissue blood flow. The relative potency of these compounds to increase this blood flow was NECA greater than ADO greater than R-PIA. Infusing R-PIA and NECA intravenously caused a fall in diastolic blood pressure and heart rate whereas an infusion of ADO up to 380 nmol/rat per min did not elicit any systemic cardiovascular effect. A 10-mg/kg dose of alkylxanthines such as caffeine, 8-phenyltheophylline (8-PT) and 8-sulfophenyltheophylline (8-SPT) inhibited the half-maximum responses of interscapular brown adipose tissue blood flow to ADO by 41, 71 and 65% respectively indicating that caffeine is less potent than 8-PT and 8-SPT. The ADO uptake inhibitor, dipyridamole, significantly potentiated the ADO effect. These results suggest that the increased interscapular brown adipose tissue blood flow caused by ADO might possibly be mediated by A2 receptors.