Diastolic oscillation in muscle tension and length

Abstract

Recently Reiter ('61, '62a, '62b), described a mechanical phenomenon (which he called "after-contraction") in isolated surviving mammalian myocardium. This phenomenon consisted of reversible contraction-like variations in isometric tension occurring after the cessation of electrical stimulation and in the absence of detectable changes in transmembrane potential. Previously Schmidt and Chang ('61 ) had reported the occurrence of "second contractions" as an incidental side effect of rapidly lowering the temperature of isolated dog purkinje tissue. These second contractions appear to be identical with the after-contractions of Reiter. However Schmidt and Chang found that second contractions were associated with an extremely prolonged action potential and considered the phenomenon a manifestation of some toxic or pathological process associated with rapid induction of profound h ypothermia .In the course of studies in this laboratory (Smith and Katzung, '62), comparing the function of myocardium from hibernators and nonhibernating species, some of the conditions described by Schmidt and Chang and by Reiter were reproduced and very prominent contraction-like variations of tension ("oscillations") were observed following cessation of electrical excitation.However, in contrast to the previous reports, it was possible to evoke the phenomenon under a wide variety of conditions, including physiological temperatures and ionic environments and in the absence of drugs. Because of the potential significance of the phenomenon in the physiology and pharmacology of normal myocardium an investigation of some of the determinants was carried out. A preliminary report has been published (Katzung, '63). METHODSIn most of the experiments an integrated muscle chamber-transducer system was employed which incorporates RCA 5734 mechano-electronic transducer triodes (Katzung, '64). The compliance of these units permitted not more than 4 x cm shortening per gram of muscle tension. The free (unloaded) resonant frequency lay between 300 and 500 CPS. However, to eliminate any possibility of transducer artifact several experiments were carried out with Grass FT03 strain gage transducers and one with a Schilling type variable capacitance isotonic transducer (Schilling, '60). Results of these preliminary isotonic experiments were not quantitated but will be the subject of further research.The transduced muscle contractions were displayed on a Tektronix 502 oscilloscope and photographed with a Grass C4 kymograph camera or were recorded with direct writing oscillographs. Two channels were used in parallel for each preparation. One was recorded at a sensitivity suitable for recording the full deflections of the active contractions. The second channel was recorded at 2 to 50 times greater sensitivity as required for suitable display of the diastolic baseline. The recorded deflections were measured manually or with the aid of analog-to-digital converter (Benson-Lehner OSCAR K) and some data was processed in an IBM 1620 computer.Tissues were electrical...

Effects of Stimulation Frequency on Myocardial Extensibility

“…Figure 8 illustrates that aftercontractions are potentiated by increasing the stimulation frequency. This has been reported by several other workers (24)(25)(26)(27)(28). Eliciting aftercontractions by paired stimulation as shown in Figure 9 has not been previously reported.…”

“…All observers agree that aftercontractions are more prominent at temperatures below 37° C and when Ca ++ is increased in the bath (24)(25)(26)(27)(28). The rapid diastolic changes associated with aftercontractions in this study were observed at 22° C. The rapid fall in isometric diastolic tension with paired stimulation reported by Scherlag et al (20) was in a cat papillary muscle at 37° C. However, their Figure 2 shows unmistakable aftercontractions at 37° C. Figure 1 in their paper does not have sufficient time resolution to determine if aftercontractions are present.…”

“…The rapid diastolic changes associated with aftercontractions in this study were observed at 22° C. The rapid fall in isometric diastolic tension with paired stimulation reported by Scherlag et al (20) was in a cat papillary muscle at 37° C. However, their Figure 2 shows unmistakable aftercontractions at 37° C. Figure 1 in their paper does not have sufficient time resolution to determine if aftercontractions are present. Katzung (28) reports aftercontractions at 35° C with increased Ca". In one of the isotonic preparations studied at 37° C, the Caw as increased tio n w i t h aftercontractions were elicited by paired stimulation similar to the experiment shown in Figure 9.…”

“…Aftercontractions disappear at low stimulation frequencies (24)(25)(26)(27)(28). It was not possible to obtain the right …”

“…Spontaneous aftercontractions are very important in the interpretation of the results; 448 FEIGL they have been observed thus far only in isolated in-vitro myocardial preparations (24)(25)(26)(27)(28). They are distinct, small, slow contractions that occur in the absence of a propagated action potential and are most prominent at temperatures below 37° C. Experiments were done at 22° C to study the effects of aftercontractions.…”