Sodium extrusion by internally dialyzed squid axons

A method has been developed which allows a length of electrically excitable squid axon to be internally dialyzed against a continuously flowing solution of defined composition. Tests showed that diffusional exchange of small molecules in the axoplasm surrounding the dialysis tube occurred with a half-time of 2-5 min, and that protein does not cross the wall of the dialysis tube. The composition of the dialysis medium was (mM): K isethionate 151, K aspartate 151, taurine 275, MgCI(2) 4-10, NaCl 80, KCN 2, EDTA 0.1, ATP 5-10, and phosphoarginine 0-10. The following measurements were made: resting Na influx 57 pmole/cm(2)sec (n = 8); resting potassium efflux 59 pmole/ cm(2)sec (n = 4); stimulated Na efflux 3.1 pmole/cm(2)imp (n = 9); stimulated K efflux 2.9 pmole/cm(2)imp (n = 3); resting Na efflux 48 pmole/cm(2)sec (n = 18); Q(10) Na efflux 2.2 (n = 5). Removal of ATP and phosphoarginine from the dialysis medium (n = 4) or external application of strophanthidin (n = 1) reversibly reduced Na efflux to 10-13 pmole/cm(2)sec. A general conclusion from the study is that dialyzed squid axons have relatively normal passive permeability properties and that a substantial fraction of the Na efflux is under metabolic control although the Na extrusion mechanism may not be working perfectly.

Influence of the thyroid state on the intrinsic contractile properties and energy stores of the myocardium

The intrinsic contractile properties of isolated cat papillary muscles and myocardial high energy phosphate stores were examined at three levels of thyroid activity and correlated with hemodynamic measurements in the intact animal. In addition, the relationship of thyroid state to endogenous norepinephrine stores and myocardial responsiveness to certain inotropic interventions were studied. In muscles from hyperthyroid cats, the velocity of shortening and the rate of tension development were markedly augmented, while duration of active state was decreased, compared to euthyroid muscles. These findings occurred in the presence and absence of intact norepinephrine stores and over a wide range of temperature and contraction frequency. The opposite changes occurred in muscles from hypothyroid cats. Isometric tension was slightly higher in muscles from hyperthyroid and lower in muscles from hypothyroid cats. The inotropic response to both norepinephrine and strophanthidin varied inversely with the level of thyroid state and allowed all three groups of muscles to reach a common ceiling of isometric tension regardless of thyroid state. Creatine phosphate and adenosine triphosphate stores were intact at all three levels of thyroid state. Thus, the level of thyroid activity profoundly affects the intrinsic contractile state of cardiac muscle, independent of both norepinephrine stores and alterations in high energy phosphate stores, and, in addition, modifies the responsiveness of cardiac muscle to inotropic agents.

Role of acetylstrophanthidin in augmenting myocardial oxygen consumption. Relation of increased O-2 consumption to changes in velocity of contraction

A polarographic method was used to determine the effects of acetylstrophanthidin on myocardial oxygen consumption (MV · O 2 ) of 19 cat papillary muscles contracting under both afterloaded and isometric conditions. Under afterloaded conditions, acetylstrophanthidin shifted the force-velocity relation to the right and produced increments in both the extent and velocity of shortening at constant levels of developed force. These changes in myocardial mechanical behavior after strophanthidin were always associated with an increased MV · O 2 . Since both the extent and velocity of shortening increased following augmentation of the contractile state with glycoside, consideration of the individual effect of these variables in myocardial mechanical behavior on MV · O 2 was precluded. Therefore, experiments were performed on isometrically contracting muscle to evaluate the effect of changes in the velocity of contraction on MV · O 2 independent of changes in contractile element shortening. Comparison of the MV · O 2 of isometric contractions at equal levels of developed tension (and thus equivalent amounts of internal contractile element shortening) was accomplished by decreasing initial muscle length after augmentation of the contractile state by acetylstrophanthidin. Under these conditions, augmentation of the contractile state, characterized by an increase in the velocity of contraction, was associated with an increased MV · O 2 . Thus it is concluded that the effect of acetylstrophanthidin is to increase MV · O 2 at a constant load and that this augmentation of MV · O 2 can be related to the change in contractile state of the muscle.

Contractile state of cardiac muscle obtained from cats with experimentally produced ventricular hypertrophy and heart failure

The contractile state of papillary muscles from hypertrophied and from failing right ventricles of cats with pulmonary artery constriction was studied. In muscles from failing hearts, the maximum velocity of shortening, active length-tension curves, and maximum rate of tension development were decreased, while the passive length-tension curves and the time from stimulation to peak tension were normal. The augmentation of isometric tension achieved by paired electrical stimulation, increasing frequency of contraction, and strophanthidin was reduced. In muscle from hearts without failure but with ventricular hypertrophy, there were qualitatively similar depressions of contractile function, although of lesser magnitude. It is concluded that congestive heart failure is associated with extreme quantitative abnormalities of the intrinsic contractile state of each unit of heart muscle, which reflect a depression in the intensity of the active state. Further, ventricular hypertrophy in the absence of failure is associated with a depression of the contractility of each unit of myocardium, while cardiac compensation is maintained by the increase in muscle mass.

Cardiac sarcotubular vesicles. Effects of ions, ouabain and acetylstrophanthidin

The transport of calcium through the membranes of the sarcoplasmic reticulum plays an important role in the relaxation-contraction cycle of the heart. To study and characterize this system, fragmented sarcotubular vesicles from dog hearts were prepared by ultracentrifugation. The fraction between 15,000 and 40,000 g was partially purified by gradient centrifugation. Calcium accumulation by the vesicles in the presence of ATP, Mg, 45 Ca, and oxalate was studied. The rate of calcium uptake increased with the ATP concentration and was not affected by changes in K or Na concentration nor by substitution of isosmotic amounts of sucrose for 75% of the Na ions. Splitting of ATP was increased upon substitution of sucrose. In aged preparations of vesicles, ouabain or acetylstrophanthidin decreased the rate of their calcium accumulation and partially reversed the inhibition of myofibrillar ATPase by the vesicles. It is suggested that the partial inhibition of calcium uptake increases the concentraton of free calcium, and thus makes more calcium available for contraction of the myofibril.