Respiratory muscle injury, fatigue and serum skeletal troponin I in rat

Abstract

To evaluate injury to respiratory muscles of rats breathing against an inspiratory resistive load, we measured the release into blood of a myofilament protein, skeletal troponin I (sTnI), and related this release to the time course of changes in arterial blood gases, respiratory drive (phrenic activity), and pressure generation. After ∼1.5 h of loading, hypercapnic ventilatory failure occurred, coincident with a decrease in the ratio of transdiaphragmatic pressure to integrated phrenic activity (P di / Phr) during sighs. This was followed at ∼1.9 h by a decrease in the P di / Phr ratio during normal loaded breaths (diaphragmatic fatigue). Loading was terminated at pump failure (a decline of P di to half of steady-state loaded values), ∼2.4 h after load onset. During 30 s occlusions post loading, rats generated pressure profiles similar to those during occlusions before loading, with comparable blood gases, but at a higher neural drive. In a second series of rats, we tested for sTnI release using Western blot-direct serum analysis of blood samples taken before and during loading to pump failure. We detected only the fast isoform of sTnI, release beginning midway through loading. Differential detection with various monoclonal antibodies indicated the presence of modified forms of fast sTnI. The release of fast sTnI is consistent with load-induced injury of fast glycolytic fibres of inspiratory muscles, probably the diaphragm. Characterization of released fast sTnI may provide insights into the molecular basis of respiratory muscle dysfunction; fast sTnI may also prove useful as a marker of impending respiratory muscle fatigue.

Cardiorespiratory failure in rat induced by severe inspiratory resistive loading

“…This sequence of events leading to pump failure (Fig. 7) differs markedly from that in our previous study of the effect of a moderate load (43) in which hypercapnic failure and the release of sTnI preceded diaphragmatic fatigue by ϳ30 min. Third, diaphragmatic fatigue occurred in two stages; very rapidly, coinciding with hypercapnic failure and the detection of fast sTnI in the blood, and later almost immediately (2 min) after an abrupt fall in MAP.…”

“…The purpose of this study was to test the hypotheses that, compared with the moderate load in our previous study (43), a severe load would cause respiratory pump failure without the development of respiratory muscle injury and fatigue and do so primarily because of central failure (often called central fatigue). Contrary to the first hypothesis, injury and diaphragmatic fatigue occurred very rapidly, before pump failure.…”

“…Once a surgical plane of anesthesia was established, the rat was placed supine; body temperature was maintained at ϳ37°C with a servocontrolled heating pad. Preparation of the animals was identical to that described previously (43). In brief, after making a midline incision in the neck, a tracheal cannula was inserted; one port was connected to a pressure transducer to measure Ptr, the other to a two-way valve to separate inspiratory and expiratory flows (Hans Rudolph 2300, Kansas City, MO).…”

“…We recently developed an anesthetized rat model in which a moderate inspiratory resistive load (IRL) [60% of the peak tracheal pressure (Ptr) developed during a prior 30-s occlusion] elicits a stereotyped sequence of hypercapnic failure, muscle injury [release of skeletal troponin I (sTnI)], diaphragmatic fatigue [decreased ratio of transdiaphragmatic pressure to integrated phrenic activity (Pdi/͐Phr)], central failure (abrupt bradypnea), and respiratory pump failure (decreased Pdi and Ptr) (43). Failure occurred after 2.4 h, much longer than that reported by others in rats (6,11,34,45,47) and rabbits (39), where failure occurs in Ͻ20 min, a failure considered to be of central origin (39,49).…”

“…Samples were analyzed for sTnI by using Western blot-direct serum analysis (WB-DSA) as previously described (42,43). Anti-TnI monoclonal antibodies of confirmed isoform specificity were chosen for WB-DSA: sTnI, FI-32 and FI-23 (fast only, Spectral Diagnostics, Toronto, ON, Canada); MYNT-S (preferential for slow in rat; 31); and 3I-35 (fast, slow, and cardiac; Spectral Diagnostics).…”