Effect of Hypoxia on Myocardial Relaxation in Isometric Cat Papillary Muscle

Abstract

A B S T R A C T Myocardial relaxation is an important energy-dependent process. Hypoxia, unlike ischemia, has not been shown to impair myocardial relaxation. This difference may be because (a) the traditional index to assess isometric muscle relaxation (halftime to relaxation or RT,/2) reflects both changes in developed tension as well as relaxation and (b) the relaxation process is highly sensitive to temperature and previous papillary muscle studies have been conducted under hypothermic conditions. The present study examines the effect of hypoxia on the relaxation process of 31 isometrically contracting kitten papillary muscles at hypothermic (29°C) and euthermic (38°C) conditions using RT112, the peak rate of tension fall (-dT/dt) and -dT/dt normalized for tension ([peak -dT/dt]/T and max [-dT/dt per T]). Hypoxia at 29°C resulted in a fall in RT,/2 from 278±11 (SEM) to 230±17 ms (P < 0.01) and no change in (peak -dTIdt)/T and max (-dTldt per T). However, at 38°C, hypoxia impaired relaxation as reflected in a prolongation of RTI/2 from 101±6 to 126±8 ms (P < 0.01) in spite of a substantial fall in peak tension. Moreover, (peak -dTIdt)IT decreased from -15.4±0.7 to -11.0±0.8/s (P <0.01) and max (-dT/dt per T) decreased from -25.1±1.8 to -13.8±0.9/s (P < 0.01). In conclusion, the present study demonstrates that hypoxia impairs the relaxation process of cardiac muscle.

Factors contributing to altered left ventricular diastolic properties during angina pectoris.

“…Since ventricular relaxation involves the active uptake of calcium by the sarcoplasmic reticulum, it is not surprising that hypoxia or ischemia substantially may impair this process. 40 42 Indeed, Palacios and Powell43 and Frist et al 44 have presented strong evidence that hypoxia impairs the relaxation process of cardiac muscle both in the intact heart and the isolated papillary muscle preparation. Their studies did not permit assessment of the persistence of incomplete relaxation to end-diastole.…”

Increased left ventricular stiffness impairs filling in dogs with pulmonary emphysema in respiratory failure.

“…However, their paper antedated the work of Serizawa et al (13) who considered the rate of ventricular relaxation as well as the extent of relaxation important in the analysis of hypoxia-induced diastolic effects. The review article also preceded the work of Frist et al (9) who showed that in isolated cat papillary muscle maintained at 38°C, relaxation time was increased rather than decreased during hypoxic conditions. Frist et al (9) pointed out that because conditions were maintained hypothermic in previous studies of hypoxic papillary muscle, a decreased rate of relaxation was not observed.…”

“…In terms ofSerizawa et al (13), the increased myocardial stiffness observed during hypoxia may be indicative ofincomplete or partial ventricular relaxation (see below). Ventricular relaxation has been shown to be a dynamic, energy dependent process (9,12,13,(32)(33)(34). When the energy supply to the heart muscle is decreased, the relaxation process may be altered such that increased ventricular resting tone occurs for a given end diastolic volume.…”

“…Ventricular relaxation (9)(10)(11)(12)(13)(14) after contraction is an energy requiring process. Altered ventricular relaxation may develop when a supply-demand imbalance in myocardial oxygen occurs (9)(10)(11)(12)(13)15). In isolated heart preparations, previous investigators have shown that during anoxic conditions left ventricular diastolic pressure was increased relative to volume throughout diastole (12,13,15).…”

Chronic exercise training protects aged cardiac muscle against hypoxia.

“…Because different results concerning the effect of hypoxia on myocardial relaxation have been reported from studies conducted at different temperatures (16), we chose to perform this study at three temperatures. The left ventricle (LV)' becomes hypertrophied during senescence (6,8,(10)(11)(12)(13), and exercise conditioning alters cardiac function in hypertrophied hearts whether they are senescent or not (13,17).…”