Effects of abrupt load alterations on force—velocity—length and time relations during isotonic contractions of heart muscle: load clamping

Abstract

SUMMARY1. Abrupt alterations in load (load-clamping) have been imposed on cat papillary muscles during the course of isotonic shortening, between the onset of shortening and peak shortening.2. For any given total load, whether imposed during the course of shortening or before stimulation, the velocity of shortening is determined solely by the instantaneous length, and not by the sequence of length and tension changes through which it arrived at that length.3. This unique force-velocity-length relation is independent oftime from just after the onset of shortening until just prior to peak shortening.4. These results suggest that a steady state exists for the maximum intensity of active state in heart muscle over a major portion of the time during which isometric force is rising, and that heart muscle always senses total load while shortening.

Force-velocity relation and contractility in striated muscles.

“…SONNENBLICK [49] suggested that the maximum velocity, v0, is independent of muscle length. BRUTSAERT et al [7] also confirmed using load clamp technique that vo was constant between 0.88-1.0 Lm in cat papillary muscle. NILssoN [43], however, found that vo as well as To decreased with decreasing muscle length, so that v0/T0 was almost independent of length.…”

Influence of Caffeine and Other Inotropic Interventions on the Onset of Unloaded Shortening Velocity in Mammalian Heart Muscle

“…normally remains independent of time and thus that velocity active state remains constant throughout most of shortening (8). In the present study the timing of the onset of unloaded shortening velocity was directly measured using cat papillary muscle preparations.…”

“…Papillary muscles (Table 1) from the right ventricle of cats were mounted as previously described (7)(8)(9) with their lower end clipped to an underwater force transducer and their upper tendinous end attached by a short silk thread to the tip of a lever system.…”

“…The electromagnetic lever system consisted of a lightweight lever attached to a coil suspended in the field of a permanent magnet (7,8). Displacement of the lever was measured by a photoelectric transducer over a linear range of 2.2 mm of muscle shortening.…”

“…This procedure was carried out by load clamping, i.e., load was altered during the course of shortening (8). The muscle sensed the total instantaneous load at any moment during shortening regardless of the previous history of that contraction.…”