The equivalent circuit that has been used in the analysis of nerve voltage-clamp data is that of the membrane capacity in parallel with the membrane resistance. Voltage-clamp experiments on frog atrial tissue indicate that this circuit will not suffice for this cardiac tissue. The change in membrane current associated with a step change in membrane potential does not show a rapid spike of capacitive current as would be expected for the simple parallel resistance-capacitance network. Rather, there is a step change in current followed by an exponential decay in current with a time constant of about 1 msec. This relatively slow capacitive charging current suggests that there is a resistance in series with the membrane capacity. A possible equivalent circuit is that of a series resistance external to the parallel resistance-capacitance network of the cell membranes. Another possible circuit assumes that the series resistance is an integral part of the cell membrane. The data presented in this paper demonstrate that the equivalent circuit of a bundle of frog atrial muscle is that of an external resistance in series with the cell membranes.

The homogeneity of voltage clamp control in small bundles of frog atrial tissue under double sucrose-gap voltage clamp conditions was assessed by intracellular microelectrode potential measurements from cells in the test node region. The microelectrode potential measurements demonstrated that (1) good voltage control of the impaled cell existed in the absence of the excitatory inward currents (e.g., during small depolarizing clamp pulses of 10-15 mV), (2) voltage control of the impaled cell was lost during either the fast or slow excitatory inward currents, and (3) voltage control of the impaled cell was regained following the inward excitatory currents. Under nonvoltage clamp conditions the transgap recorded action potential had a magnitude and waveform similar to the intracellular microelectrode recorded action potentials from cells in the test node. Transgap impedance measured with a sine-wave voltage of 1,000 Hz was about 63% of that measured either by a sine-wave voltage of 10 Hz or by an action potential method used to determine the longitudinal resistance through the sucrose-gap region. The action potential data in conjunction with the impedance data indicate that the extracellular resistance (R(s)) through the sucrose gap is very large with respect to the longitudinal intracellular resistance (R(i)); the frequency dependence of the transgap impedance suggests that at least part of the intracellular resistance is paralleled by a capacitance. The severe loss of spatial voltage control during the excitatory inward current raises serious doubts concerning the use of the double sucrose-gap technique to voltage clamp frog atrial muscle.

IT IS well established that the resting tension (stress)-sarcomere length relationships of a variety of intact preparations of cardiac muscle are different from those of skeletal muscle preparations. In contrast to skeletal muscle in which resting tension is negligible at sarcomere lengths below about 2.3 jim (Gordon et al., 1966;Lannergren and Noth, 1973; Moss and Halpem, 1977), mammalian cardiac preparations have relatively high resting tensions at these sarcomere lengths (Spiro and Sonnenblick, 1964), and the resting tension increases markedly at sarcomere lengths beyond 2.3 /im. In nonmammalian cardiac preparations, variable results have been obtained. Starling (1918) found in intact tortoise ventricle that the resting tension remained relatively small over a wide range (approximately 6-fold) of diastolic volumes, which included the rising phase and the relatively broad range (approximately 2-fold) of the active tension-diastolic volume relationship. In contrast, both Winegrad (1974) and Matsubara and Maruyama (1977) found that the resting tension increased significantly over the sarcomere length range of 2.0-2.6 /xm in small bundles of frog atrial tissue. Nassar et al. (1974) found

A method is presented which is capable of continuously monitoring the degree of hemolysis in erythrocyte suspensions too dilute to be monitored by conventional light transmission techniques. Scattered light is used to non-destructively assess hemolysis in sparse monolayers which are particularly well suited to many photohemolytic studies. The small angle scattering (

Carotid sinus baroceptor responses were studied in dogs before and after administration of shock-producing quantities of E. coli endotoxin. Responses were characterized as the relationship between intrasinusal stimulus pressure and baroceptor nerve discharge frequency. Both constant and pulsating pressure stimuli were used. After endotoxin administration and the onset of endotoxin shock it was found that postendotoxin discharge frequencies exceeded control discharge frequencies for most pressures. These response shifts result in a resetting of the hemoregulatory barostatic mechanism to regulate blood pressure to a new and hypotensive level. It is concluded that in the later phases of endotoxin shock, the reflex modification may play the predominant role in maintaining hypotension and that barostatic resetting partially explains the poor compensatory response to the initial profound hypotension after endotoxin administration.