VASOCONSTRICTOR-MEDIATED RELEASE OF LACTATE FROM THE PERFUSED RAT HINDLIMB

The effects of different vasomodulators on lactate release by the constant-flow-perfused rat hindlimb were examined and compared with that by perfused mesenteric artery, incubated preparations of aortas, soleus and epitrochlearis muscles, and perifused soleus muscles. Infusion of vasopressin (0.5 nM), angiotensin II (5 nM), norepinephrine (50 nM), and methoxamine (10 muM) into the hindlimbs of 180- to 200-g rats increased the perfusion pressure by 112-167% from 30.4 +/- 0.8 mmHg, O2 consumption by 26-68% from 6.4 +/- 0.2 mumol . g-1. h-1, and lactate efflux by 148-380% from 5.41 +/- 0.25 mumol . g-1 . h-1. Hindlimbs of 100- to 120-g rats responded similarly to angiotensin II. Isoproterenol (1 muM) had no effect on O2 uptake or perfusion pressure but increased lactate release by 118%. Nitroprusside (0.5 mM) markedly inhibited the vasoconstrictor-mediated increases in lactate release, perfusion pressure, and O2 consumption by the hindlimb but had no effect on isoproterenol-mediated lactate efflux. Serotonin (6.7 muM) increased lactate release from the perfused mesenteric artery by 120% from 5.48 mol . g-1 . h-1. Lactate release by incubated aorta was increased by angiotensin II (50 nM), isoproterenol (1 muM), and mechanical stretch. The increase mediated by angiotensin II was blocked by glycerol trinitrate (2.2 muM), which had no effect on lactate release by isoproterenol. Neither angiotensin II (5 nM) nor vasopressin (0.5 nM) increased lactate release from incubated soleus and epitrochlearis muscles; however, lactate release was increased by isoproterenol, and this increase was unaffected by glycerol trinitrate (2.2 muM). Similarly, neither norepinephrine (0-0.1 muM) nor angiotensin (0-0.1 muM) altered the O2 consumption and lactate efflux by perifused soleus muscles. Taken together, these findings suggest that, in the perfused rat hindlimb, the increase in lactate efflux during agonist-mediated vasoconstriction is of vascular smooth muscle origin and may constitute a significant fuel for whole body thermogenesis.