In the present study, the effects of angiotensin II (ANG II) on tension development and associated metabolism during twitch and tetanic stimulation via the sciatic nerve of the gastrocnemius-plantaris-soleus muscle group of the perfused rat hindlimb were investigated. Rat hindlimbs were perfused at constant flow with an erythrocyte-containing medium equilibrated with 95% air-5% CO2 at 37 degrees C, and determinations of oxygen and glucose uptake, lactate and glycerol release, and 2-deoxy-D-[1-3H]glucose uptake (Rg') into individual muscles were carried out. ANG II (1 nM) infusion alone caused vasoconstriction with increased oxygen (55%) and glucose (98%) uptake and lactate (37%) and glycerol (64%) release. ANG II infusion during muscle contraction gave less vasoconstriction but increased the tension development during tetanic stimulation by 80% and increased the contraction-induced oxygen uptake and Rg' by plantaris and gastrocnemius red and white muscles. These effects of ANG II may have been due to increased nutritive flow to contracting muscles or to redirection of flow from noncontracting and type I fiber muscles to the type II fiber contracting muscles in the hindlimb. The results indicate that the regulation of flow by the vasculature is an important regulator of muscle contraction and metabolism.

Exogenous substrates for capillary endothelial enzymes have potential as markers for changes in capillary recruitment (albeit nutritive flow). The metabolism of infused 1-methylxanthine (1-MX) to 1-methylurate (1-MU) by capillary endothelial xanthine oxidase of the constant-flow perfused rat hindlimb was shown previously to decrease with oxygen uptake (VO2) when nutritive flow was decreased. In the present study, the metabolism of 1-MX was investigated under conditions when VO2 and nutritive flow are known to increase during muscle contraction. The constant-flow red blood cell-perfused rat hindlimb at 37 degrees C was used with sciatic nerve stimulation, and perfusate samples from whole hindlimb and working muscles taken for analysis of oxygen, lactate, 1-MX and 1-MU. Flow to muscle was assessed separately using fluorescent microspheres and was found to increase 2.3-fold to the working muscles while flow to the non-working leg muscles decreased to compensate. The activity of xanthine oxidase of whole muscle extracts was not altered by contraction. Samples from the vein draining the working muscles, and microsphere measurements of flow, indicated increased VO2 (5.5-fold to 249.2 +/- 43.1 micromol h-1 g-1, P < 0.001), and 1-MX conversion (2.5-fold to 1.87 +/- 0.25 micromol h-1 g-1, P < 0.01) (SEM are shown). It is concluded that as 1-MX metabolism parallels VO2, this substrate may be a useful indicator of changes in capillary (nutritive) surface area in muscle.

The vasoconstrictors norepinephrine (NE) and angiotensin II (AII) mediate increases in oxygen uptake (VO2) by the constant flow perfused rat hind limb that are inhibited by quinidine-like membrane-stabilizing effects (involving the interruption of action potential) of (+/-)-propranolol with little effect on vasoconstriction. The membrane labilizer veratridine, 10 microM, which has the capability of maintaining voltage-gated Na+ channels of the plasma membrane in their open state, also increases VO2 but without an increase in pressure. Thus in the present study veratridine was characterized in detail and compared with NE in the same system. Veratridine (3-100 microM) produced a dose-dependent stimulation of VO2 (from 11.8 +/- 0.3 to 20.4 +/- 0.6 mumol.h-1.g-1 (n = 5), p < 0.0001) and lactate efflux (LE) (from 7.4 +/- 0.6 to 23.0 +/- 4.7 mumol.h-1.g-1 (n = 5), p < 0.01). These increases were independent of vasoconstriction at low doses (< or = 10 microM). At higher doses of veratridine the accompanying minor vasoconstriction (from 17 +/- 1 to 30 +/- 2 mmHg (1 mmHg = 133.3 Pa) (n = 5), p < 0.005) was blocked by sodium nitroprusside (NP) while neither VO2 nor LE was greatly affected. Low Na+ perfusions (25 mM) did not affect the vasoconstrictor action of NE but markedly inhibited increases in VO2 and LE due to either veratridine or NE. Veratridine (10 microM) mediated increases in VO2 and LE were blocked by either (+/-)-propranolol (100 microM) or 150 microM quinidine. It is concluded that vasoconstrictors such as NE, which stimulate VO2 in the perfused rat hind limb, do so by a two-stage process involving an essential nitroprusside-sensitive redirection of flow followed by a mechanism involving increased ion movement across skeletal muscle cell membranes, which is blocked by membrane stabilizers. Veratridine achieves a similar increase in VO2 but may do so by directly destabilizing the skeletal muscle cell membrane without the requirement of a redirection of flow.

In the constant flow perfused rat hind limb, norepinephrine (NE) evoked increases in oxygen uptake (VO2) and lactate efflux (LE) were inhibited by the cardiac glycoside ouabain (1 mM), without interrupting the NE-mediated vasoconstriction. The membrane labilizer veratridine, previously shown to increase VO2 and LE, without increasing perfusion pressure, was also shown to be inhibited by the cardiac glycoside ouabain, as well as by the ouabain analogues digitoxin and digoxin. The stimulatory actions of veratridine on VO2 were inhibitable by low doses of the specific sodium channel blocker tetrodotoxin (TTX), while NE effects were unaffected, suggesting that NE may be acting via a TTX-insensitive sodium channel. It is concluded that agents such as NE (a vasoconstrictor) or veratridine (a membrane labilizer), which stimulate VO2 in the perfused rat hind limb, do so by increasing Na+ influx. The observed increases in oxygen consumption and LE are due to Na+-K+ ATPase activity to pump Na+ out of the cell at the expense of ATP turnover. Energy dissipation due to Na+ cycling may be a form of facultative thermogenesis attributable to NE that can be stimulated by membrane labilizers such as veratridine in the constant flow perfused rat hind limb.

The vasoconstrictors norepinephrine (NE) and angiotensin II (AII) mediate increases in oxygen uptake (VO2) by the constant flow perfused rat hind limb that are inhibited by quinidine-like membrane-stabilizing effects (involving the interruption of action potential) of (+/-)-propranolol with little effect on vasoconstriction. The membrane labilizer veratridine, 10 microM, which has the capability of maintaining voltage-gated Na+ channels of the plasma membrane in their open state, also increases VO2 but without an increase in pressure. Thus in the present study veratridine was characterized in detail and compared with NE in the same system. Veratridine (3-100 microM) produced a dose-dependent stimulation of VO2 (from 11.8 +/- 0.3 to 20.4 +/- 0.6 mumol.h-1.g-1 (n = 5), p < 0.0001) and lactate efflux (LE) (from 7.4 +/- 0.6 to 23.0 +/- 4.7 mumol.h-1.g-1 (n = 5), p < 0.01). These increases were independent of vasoconstriction at low doses (< or = 10 microM). At higher doses of veratridine the accompanying minor vasoconstriction (from 17 +/- 1 to 30 +/- 2 mmHg (1 mmHg = 133.3 Pa) (n = 5), p < 0.005) was blocked by sodium nitroprusside (NP) while neither VO2 nor LE was greatly affected. Low Na+ perfusions (25 mM) did not affect the vasoconstrictor action of NE but markedly inhibited increases in VO2 and LE due to either veratridine or NE. Veratridine (10 microM) mediated increases in VO2 and LE were blocked by either (+/-)-propranolol (100 microM) or 150 microM quinidine. It is concluded that vasoconstrictors such as NE, which stimulate VO2 in the perfused rat hind limb, do so by a two-stage process involving an essential nitroprusside-sensitive redirection of flow followed by a mechanism involving increased ion movement across skeletal muscle cell membranes, which is blocked by membrane stabilizers. Veratridine achieves a similar increase in VO2 but may do so by directly destabilizing the skeletal muscle cell membrane without the requirement of a redirection of flow.