1. We tested the hypotheses that coupling between oxidative metabolism and force in noradrenaline (NOR)-activated rabbit aorta is controlled (a) by an energy-dependent step or steps in receptor-operated coupling mechanisms upstream to myosin light chain (MLC) kinase, or (b) by energy limitation of MLC kinase-mediated phosphorylation of the MLC or actin-activated myosin ATPase. 2. Oxidative energy production was rapidly inhibited by decreasing organ bath P(O2) to < 30 mmHg. There was no difference, comparing KCl- or NOR-induced force, in the rates of decrease of [PCr] (phosphocreatine) or [ATP] following inhibition of oxidative energy production. (In this report we use the term [PCr] and [ATP] to indicate mean tissue values). Initial rates of decrease in [PCr] and [ATP] following inhibition of oxidative energy production were 0.05 mM/min and 0.06 mM/min, respectively. 3. Despite similar decreases in mean tissue [PCr] and [ATP], relaxations of KCl-induced contractions following inhibition of oxidative energy production were markedly delayed and were blunted compared to relaxations seen during NOR-induced contractions. The threshold mean tissue [PCr] and [ATP] for relaxation during KCl stimulation were 0.25 and 0.60 to 0.80 mM, respectively. During NOR stimulation, threshold values of [PCr] and [ATP] were 0.50 mM and 0.80 mM, respectively. Mean tissue [PCr] and [ATP] levels at 50% relaxation of KCl-induced force were < 0.1 mM and 0.1 mM, respectively. Fifty per cent relaxation of NOR-induced force occurred at [PCr] and [ATP] values of 0.35 mM and 0.65 mM, respectively. 4. MLC phosphorylation levels decreased during relaxation of NOR force evoked by inhibition of oxidative energy production. There was no change in the level of MLC phosphorylation following inhibition of oxidative energy production in KCl-contracted muscle even at mean tissue [PCr] and [ATP] lower than values associated with decreases in MLC phosphorylation during relaxations of NOR-induced force. 5. Oxygen-induced redevelopment of force during NOR exposure was not dependent on extracellular Ca2+. Mean tissue [PCr] increased prior to onset of O2-evoked force redevelopment. Increases in MLC phosphorylation were seen at the time of onset of force redevelopment. 6. Oxidative metabolism-contraction coupling during NOR-stimulation seems not to be due to energy limitation of the MLC kinase reaction or actin-activated myosin ATPase. Data suggest the rate-limiting step is an energy-dependent reaction in receptor-operated coupling mechanisms upstream to MLC kinase. The rate-limiting energy-dependent reaction during KCl contractions appears to be actin-activated myosin ATPase. Data are consistent with actin-activated myosin ATPase being inhibited by 50% at mean tissue [ATP] of approximately 0.1 mM and MLC kinase being inhibited by 50% at an [ATP] of < 0.1 mM. The rate-limiting step for oxidative metabolism-contraction coupling during NOR-stimulation is inhibited by 50% at 0.65 mM [ATP].
