ISSN:
1432-2013
Keywords:
Force-velocity relation
;
Vascular smooth muscle
;
Portal vein
;
Calcium
;
Temperature
;
Contractility
;
Staircase phenomenon
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Abstract The velocity of the contractile elementV CE of the rat portal vein during a single isometric twitch was calculated from the ratio of the rate of tension increase (dT/dt) to the stiffness (dT/dL) of the series elastic element. This stiffness was considered as a linear function of tension (dT/dL=k · T), and thus the respective term wasV CE=(dT/dt)/(kT). A polynome of the 10th order was fitted to the time course of tension change within the first seconds of stimulation. From this function, the instantaneous values of forceT, rate of tension increasedT/dt, as well as the ratio(dT/dt)/T were computed. The series elasticity was treated as the respective stiffness factor ‘k’.V max isV CE extrapolated to zero load, and this value gives an index of the turnover rate of the myosin cross-bridges. The experiments were carried out at different temperatures (37° C, 30° C, and 25° C) as well as at different intracellular calcium levels occurring as a staircase phenomenon in the first contractions after a period of non-stimulation of 20 min. With increasing temperature,T remained constant but there was an increase inV max (Q 10=1.9), peakV CE (Q 10=1.8) and in (dT/dt)max (Q 10=1.8). Furthermore, this so-called ‘tachytropic’ effect of temperature showed a reduction of the time to the maximumdT/dt (Q 10=1.4), and to the peakV CE (Q 10=1.2). During a staircase cycle the parameters describing the contractile state, i.e. theV max, the time to maximumdT/dt, and the time to peakV CE remained constant.T, maximumdT/dt and peakV CE were, however, considerably increased. These results are typical of the socalled ‘polytropic’ effect of calcium. The experimental results obtained from the isometric force-velocity relation were compared with those calculated by means of isotonic contractions in previous experiments. There were similar changes in the dynamics of contraction, irrespective of the method used for calculation ofV max; this was the case in experiments with varied temperatures and calcium levels. Therefore the determination ofV max from a single isometric contraction seems to be a suitable method of describing the elementary process of contraction in vascular smooth muscle. This method showed a higher time resolution as compared with other methods using isotonic contractions.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00581955
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