Oxygen Consumption and Lactate Production of the Rat Portal Vein in Relation to its Contractile Activity

Energy turnover in the isolated rat portal vein was investigated by measurement of oxygen consumption (Jo₂) and lactate production (J_LA) under simultaneous recording of mechanical activity. In spontaneous activity under aerobic conditions and at optimal muscle length Jo₂ and J_LA were 0.55 and 0.62/μmol/min g. respectively, corresponding to an ATP-production of 4.3 μmol/min g. When muscle length was changed, an approximately linear relation was found between energy turnover and mean isometric tension. The tension-independent part of ATP-production was 3.0 μmol/min g. In Ca²+-free solution the metabolic rate was 20 % lower still. Jo₂ was nearly equal in isometric contractions and in afterloaded isotonic contractions from the same initial muscle length. During a maximal tonic contracture in K+-depolarized portal vein Jo₂ increased to about twice that in spontaneous activity. Changes in contracture force by variations in muscle length or in [Ca²+]₀ were associated with identical linear relations between Jo₂ and active tension. This relation was less steep than the corresponding relation for spontaneous activity. The anaerobic lactate production of the portal vein was 2.7 times the aerobic level. The accelerated glycolysis did not compensate for eliminated oxidative metabolism. Under substrate-free aerobic conditions no lactate was produced by the muscle and compared to the control situation Jo₂ declined more than could be accounted for by reduced mechanical aevtivity alone. The metabolic turnover rate in relation to isometric tension is high in the rat portal vein compared to that of tonic vascular smooth muscle from larger vessels. This correlates with differences in dynamic mechanical properties. At comparable tension levels in the portal vein, the rate of cross-bridge turnover may be higher in spontaneous phasic activity than in sustained contracture.