The abnormal mechanical strength-interval curve of hypoxic rabbit papillary muscle.

The effect of hypoxic injury on the mechanical strength-interval curve of rabbit myocardium was studied, using an isolated left ventricular thick papillary muscle preparation. Histochemical staining disclosed a central core of injured tissue enveloped by viable muscle which was sustained by aerobic metabolism, being unaffected by iodoacetate. Mechanical strength-interval curves were derived by measurement of developed isometric tension during various pacing protocols. The phasic recovery of contractile tension, described by the strength-internal curve, was markedly prolonged and partly responsible for reduced tension development. Parameters of the strength-interval curve were determined by linear and monoexponential curve-fitting procedures during various experimental interventions. These parameters indicated that the contractility recovery rate was not altered by acidosis or propranolol, but the early phase of the strength-interval curve (at short test intervals) was augmented by propranolol, and the plateau of the full curve was depressed by acidosis. These observations emphasize that the strength-interval curve must be considered in any study of the effect of a drug or other intervention on the contractility of hypoxic myocardium.