The cardiovascular effects of microinjections of two gamma-aminobutyric acid (GABA) antagonists specifically acting on GABAB receptors, 2-hydroxy-saclofen (2-OH-S) and CGP-35348, into vasopressor sites of rostral ventrolateral medulla (RVLM) were studied in rats anesthetized with urethan. Bilateral microinjection of 2-OH-S induced significant increases in mean arterial pressure (MAP) and heart rate (HR) in 21 of 26 RVLM vasopressor sites (81%); average increases obtained in the 26 sites studied were +25.2 +/- 3.0 mmHg and +12.7 +/- 2.1 beats/min. Bilateral microinjection of CGP-35348 induced significant increases in MAP and HR in 10 of 12 RVLM sites (83%). Average increases in the 12 sites studied were +27.6 +/- 3.9 mmHg and +14.6 +/- 2.5 beats/min. Sixteen rats received unilateral electrolytic lesions of one RVLM. Microinjections of either 2-OH-S or CGP-35348 into vasopressor sites within the intact RVLM significantly antagonized the depressor responses observed after injections of baclofen (20 pmol) into the same sites, whereas both GABAB antagonists did not affect the depressor response induced by microinjection of muscimol (5 pmol). These results suggest a tonic inhibitory mechanism within the RVLM mediated by GABAB receptors involved in central cardiovascular regulation.

The values of membrane action potentials and maximum depolarization rates of single muscle fibers in normal Tyrode solution and in low sodium solutions containing as little as 20 per cent of the sodium chloride were measured with intracellular microelectrodes. Under these conditions the membrane potential remains unchanged up to 36 per cent of [Na+]ou, concentration, whereas the overshoot of the action potential varies linearly with the logarithm of the external sodium concentration. The maximum depolarization rate is a linear function of the external sodium concentration. The results obtained support the ionic theory for sodium and the independence principle for sodium current related to the external sodium concentration.T h e validity of the ionic theory for the striated fiber of frog's sartorius muscle was d e m o n s t r a t e d by Nastuk a n d H o d g k i n (1). T h e purpose of our research was to ascertain the validity of such theory with regard to [Na +] in the striated muscle fiber of mammals. This was done by measuring the action potential a n d the m a x i m u m rate of depolarization at different external concentrations of sodium ions. M E T H O DAll experiments were carried out on the obliquus internus abdominis muscle of guinea pigs. The muscle was excised from the animal, transferred to a plexiglas cell, and irrigated with Tyrode solution. The solution was thermostatically maintained at 37.5°C and aerated with 95 per cent oxygen and 5 per cent carbon dioxide. The composition of the solution (Tyrode, 1910)

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