ENERGETIC COST OF ACTIVATION PROCESSES DURING CONTRACTION OF SWINE ARTERIAL SMOOTH-MUSCLE

1. The objective of this study was to partition the increase in ATP co nsumption during contraction of swine carotid arterial smooth muscle e stimated from suprabasal oxygen consumption (suprabasal J(O2)) and lac tate release (J(lactate)) into a component associated with cross-bridg e cycling (J(X)) and one reflecting activation (J(A)). 2. Two experime ntal approaches - varying length under constant activation, and varyin g activation at a long length (1.8 times the optimal length for force development (L-0)) where force generation is minimal - revealed a line ar dependence of J(O2) and activation energy (J(A)) on cross-bridge ph osphorylation. Protocols inducing a large increase in myosin regulator y Light chain (MRLC) phosphorylation at 1.8L(0) resulted in significan t elevations of J(O2) and marked reductions in the economy of force ma intenance. Our evidence suggests that this is primarily due to the inc reased cost of cross-bridge phosphorylation. 3. The extrapolated estim ate of J(A) during maximal K+-induced depolarization made by varying l ength was 16%, while at 1.8L(0) it was 33% of the suprabasal J(O2) at L-0. Calculated activation energies ranged from 17 to 45% of the supra basal J(O2) at L-0 and from 72 to 87% of the suprabasal J(O2) at 1.8L( 0) under stimulation conditions that varied steady-state MRLC phosphor ylation from 1.5 to 50%. 4. The results suggest that the kinetics of c ross-bridge phosphorylation-dephosphorylation can rival those of cross -bridge cycling during isometric contractions in swine arterial smooth muscle.