Snelling, E.Seymour, R.Green, J.Meyer, L.Fuller, A.Haw, A.Mitchell, D.Farrell, A.Costello, M.Izwan, A.Badenhorst, M.Maloney, S.2017-03-152017-03-152016Journal of Applied Physiology, 2016; 121(4):900-9098750-75871522-1601http://hdl.handle.net/2440/103864Left ventricular external mechanical work rate was calculated from cardiac output and systemic mean arterial blood pressure in resting sheep (Ovis aries; N = 4) and goats (Capra hircus; N = 4) under mild sedation, followed by perfusion-fixation of the left ventricle, and quantification of the cardiac capillary-tissue geometry and cardiomyocyte ultrastructure. The investigation was extended to heavy exercise by increasing cardiac work according to published hemodynamics during sustained treadmill exercise. Left ventricular work rate averaged 0.017 W cm(-3) of tissue at rest, and was estimated to increase to ~0.060 W cm(-3) during heavy exercise. We predicted that oxygen consumption increases from 195 nmol O2 s(-1) cm(-3) at rest, to ~600 nmol O2 s(-1) cm(-3) during heavy exercise, which is within 90% of the demand rate and consistent with work remaining predominantly aerobic. Mitochondria represent 21 - 22% of cardiomyocyte volume and consume oxygen at a rate of 1150 nmol O2 s(-1) cm(-3) of mitochondria at rest, and ~3600 nmol O2 s(-1) cm(-3) during heavy exercise, which is within 80% of maximum in vitro rates and consistent with mitochondria operating near their functional limits. Myofibrils represent 65 - 66% of cardiomyocyte volume, and according to a Laplacian model of the left ventricular chamber, generate peak fiber tensions between ~54 and 62 kPa at rest and during heavy exercise, which is less than maximum tension of isolated cardiac tissue (120 - 140 kPa), and is explained by an apparent reserve capacity for tension development built into the left ventricle.enCopyright © 2016 the American Physiological SocietyCapillaryJournal article003005395310.1152/japplphysiol.00435.20160003896506000102-s2.0-84992377609263218Seymour, R. [0000-0002-3395-0059]Green, J. [0000-0001-5061-9563]