Biomedical subjects

S R Caplan

Publications and source records attributed to S R Caplan.

100 records · Page 6Linked to original sources

PubMed1968

Autonomic energy conversion. II. An approach to the energetics of muscular contraction.

All discussions of muscle energetics concern themselves with the Hill force-velocity relation, which is also the general output relation of a class of self-regulated energy converters and as such contains only a single adjustable parameter -the degree of coupling. It is therefore important to see whether in principle muscle can be included in this class. One requirement is that the muscle should possess a working element characterized by a dissipation function of two terms: mechanical output and chemical input. This has been established by considering the initial steady phase of isotonic and isometric tetanic contraction to represent a stationary state of the fibrils (a considerable body of evidence supports this). Further requirements, which can be justified for the working element, are linearity and incomplete coupling. Thus the chemical input of the muscle may be expected to follow the inverse Hill equation (see Part I). The relatively large changes in activities of reactants which the equation demands could only be controlled by local operation of the regulator, and a scheme is outlined to show how such control may be achieved. Objections to this view recently raised by Wilkie and Woledge rest on at least two important assumptions, the validity of which is questioned: (a) that heat production by processes other than the immediate driving reaction is negligible, which disregards the regulatory mechanism (possibly this involves the calcium pump), and (b) that the affinity of the immediate driving reaction is determined by over-all concentrations. The division of heat production into "shortening heat" and "maintenance heat" or "activation heat" is found to be arbitrary.

Autonomic Nervous System↗

PubMed1968

Energetics of active transport processes.

Discussions of active transport usually assume stoichiometry between the rate of transport J(+) and the metabolic rate J(r). However, the observation of a linear relationship between J(+) and J(r) does not imply a stoichiometric relationship, i.e., complete coupling. Since coupling may possibly be incomplete, we examine systems of an arbitrary degree of coupling q, regarding stoichiometry as a limiting case. We consider a sodium pump, with J(+) and J(r) linear functions of the electrochemical potential difference, -X(+), and the chemical affinity of the metabolic driving reaction, A. The affinity is well defined even for various complex reaction pathways. Incorporation of a series barrier and a parallel leak does not affect the linearity of the composite observable system. The affinity of some region of the metabolic chain may be maintained constant, either by large pools of reactants or by regulation. If so, this affinity can be evaluated by two independent methods. Sodium transport is conveniently characterized by the open-circuit potential (Deltapsi)(I=0) and the natural limits, level flow (J(+))(X+=0), and static head X(0) (+) = (X(+))(J+=0). With high degrees of coupling -X(0) (+)/F approaches the electromotive force E(Na) (Ussing); -X(0) (+)/F cannot be identified with ((RT/F) ln f)(X+=0), where f is the flux ratio. The efficiency eta = -J(+)X(+)/J(r)A is of significance only when appreciable energy is being converted from one form to another. When either J(+) or -X(+) is small eta is low; the significant parameters are then the efficacies epsilon(J+) = J(+)/J(r)A and epsilon(X+) = -X(+)/J(r)A, respectively maximal at level flow and static head. Leak increases both J(+) and epsilon(J+) for isotonic saline reabsorption, but diminishes -X(0) (+) and epsilon(Xfemale symbol). Electrical resistance reflects both passive parameters and metabolism. Various fundamental relations are preserved despite coupling of passive ion and water flows.

Biological Transport↗

PubMed1968