Cardiac myocyte function and left ventricular strains after brief ischemia and reperfusion in rabbits

After a brief episode of ischemia, myocardial function may be depressed for prolonged periods despite reperfusion. The mechanisms of postischemic dysfunction differ depending on the experimental model. Regional ischemia and reperfusion in the intact animal provide a clinically relevant model, but experimental variables are difficult to control. Experimental conditions can be well controlled in isolated cardiac muscle and myocyte preparations, but these models are limited by the assumptions used to mimic ischemia and reperfusion. This study combines the unique advantages of both preparations. We characterized in vivo alterations in regional two-dimensional finite strains with ischemia and reperfusion produced in the intact animal, then isolated cardiac myocytes from the region with postischemic dysfunction to characterize in vitro function of postischemic myocytes.

In seven anesthetized rabbits, three piezoelectric crystals were inserted in a triangular array to measure two-dimensional finite strains around the large coronary artery in the left ventricular anterior free wall. After 15 minutes of ischemia and reperfusion, strains were depressed at a stable level approximately 30% to 40% below control values between 1 and 6 hours after reperfusion. The direction of maximal shortening deformations was midway between circumferential and longitudinal directions during control and did not shift after reperfusion. In a second group of five rabbits, cardiac myocytes were isolated from the region with postischemic dysfunction after 15 minutes of ischemia and 45 minutes of reperfusion. We compared in vitro function in 45 postischemic myocytes with 48 cardiac myocytes isolated from five normal rabbits. Each rabbit (postischemic and control) contributed 9 +/- 1 (SD) myocytes to the study. All myocytes were studied within 1 hour after myocyte isolation (approximately 3 to 5 hours after reperfusion for postischemic myocytes). Myocytes were stimulated at 0.5 Hz and perfused with 2 mmol/L [Ca2+] Tyrode's solution to measure unloaded cell shortening. There was significantly less shortening in postischemic myocytes (12.4 +/- 2.1%) than control myocytes (16.2 +/- 1.2%). Maximal cell length (Lmax) was significantly longer in postischemic (134 +/- 7 microns) than control myocytes (122 +/- 7 microns), as was minimum cell length (Lmin) (118 +/- 8 versus 103 +/- 9 microns, respectively). The duration of shortening (time from stimulation to Lmin) was significantly shorter in postischemic (279 +/- 56 milliseconds) than control myocytes (405 +/- 44 milliseconds). Peak rates of cell shortening (-dL/dt) and lengthening (+dL/dt) did not differ.

In rabbits, 15 minutes of ischemia produced a stable depression in finite strains for 1 to 6 hours after reperfusion, with shortening deformations reduced by approximately 30% to 40% without a shift in direction. Cardiac myocytes isolated from postischemic myocardium display functional impairments in vitro similar to those measured in vivo, with an approximately 25% reduction in unloaded myocyte shortening and decreased contraction duration. This indicates that ischemia and reperfusion induce intrinsic impairments in contractility independently of external loading conditions. This model may be useful for examining cellular mechanisms of postischemic myocardial dysfunction.