[Contraction-relaxation dynamics and mechanical restitution in the developing myocardium of the chick embryo]. / Dinamika tsikla sokrashchenie-rasslablenie i mekhanicheskaia restitutsiia v razvivaiushchemsia miokarde émbriona tsyplenka.

Parameters of the contraction-relaxation cycle and mechanical restitution (MR) were assessed in isolated ventricular preparations of 3- and 4-day chick embryos (EM) and posthatched (PH) chicks. Ryanodine reduced the relaxation rate in the EM but increased it in the PH chicks. It also suppressed a rest-induced potentiation and the MR in all the preparations. Low Na superfusion significantly suppressed the relaxation and decreased the rest potentiation in the myocardial preparations all ages. The findings substantiate existence of a sarcolemmal Ca pool which participates in regulation of twitch parameters and Ca outflux via the Na-Ca exchange.

[The role of intra- and extracellular calcium sources in the rhythmic inotropic regulation of isometric contractions in the chick embryo myocardium]. / Rol' vnutri- i vnekletochnykh istochnikov kal'tsiia v ritmoinotropnoi reguliatsii izometricheskikh sokrashchenii miokarda émbriona tsyplenka.

Isometric contractions and force-frequency relationships (FFR) were assessed in isolated ventricular preparations of 3- and 4-day chick embryos (EM) and posthatched (PH) chicks. Pacing protocols for the FFR assessment were applied in normal buffer, buffer with 50% and 25% of normal Na+ concentration, and in buffer with ryanodine. PH myocardium showed greater peak force and more prominent FFR than EM. 50% low Na+ superfusion induced positive inotropic effect and increase in the FFR in PH and EM. 25% low Na+ superfusion induced negative inotropic effect and suppressed FFR more prominent in EM. Ryanodine more suppressed FFR in PH myocardium. The observed age-dependent differences substantiate the fact that Na-Ca exchange plays a leading role in the FFR regulation in early embryonic myocardium and sarcoplasmic reticulum plays a leading role in PH myocardium.

[Effect of temperature on the myocardial chrono- and inotropism of warm-blooded animals]. / Issledovanie vliianiia temperatury na khronoinotropiiu miokarda teplokrovnykh.

The influence of temperature on interval-force characteristics of rabbit ventricular myocardium was studied in conditions of random Gaussian sequences stimulation intervals. The interval-force relationships were described with Volterra series and cross-correlation function. Decrease of temperature causes inverse influence of Tn and Tn-1 intervals of the contraction force: at 35 degrees C coefficient of cross-correlation rho (0) of Tn-Pn equals 0.1 and coefficient of cross-correlation Tn-1-Pn rho (+1) equals -0.75; at 20 degrees C rho (0) = 0.8; rho (+1) = -0.15. Index Q10 for the process responsible for extrasystolic potentiation is about 3.0 and Q10 for restitutions process equals 1.5. At high temperature calcium seems to enter the myoplasm from extracellular pool, to be absorbed by sarcoplasmic reticulum and released in next cycle contraction. At low temperature, a part of calcium entering from that pool directly activates contractions at the same cycle.