Isolated perfused rabbit hearts have been used to determine the reflection coefficients, ~r, of the heart capillaries to certain lipoid-insoluble substances. This was clone by initially perfusing the heart with a Ringer solution containing no test molecule and then suddenly switching to a solution which differed from the original only by containing a small amount of test substance. This produced a loss of weight of the heart which was continuously recorded as a function of time. Taking the "zero" time rate of weight change and using an equation given by Kedem and Katchalsky reflection coefficients for urea, sucrose, raffinose, and inulin were obtained. These turned out to be 0.1, 0.3, 0.38, and 0.69 respectively. Using the approach of Durbin and Solomon equivalent pore radii were estimated to be about 35 Angstroms.

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We investigated voltage-dependent Ca2+ channels of bovine adrenal medulla endothelial cells with the whole cell version of the patch-clamp technique. Depolarization elicited an inward current that was carried by Ca2+ and was composed of a transient (T) current, present in all the cells tested, and a sustained (L) current, present in 65% of them. We separated these currents and measured their individual kinetic and gating properties. The activation threshold for T current was approximately −50 mV, and its maximum amplitude was −49.8 ± 4.8 pA (means ± SE, n = 19) at 0 mV. The time constant was 10.2 ± 1.5 ms ( n= 4) for activation and 18.4 ± 2.8 ms ( n = 4) for inactivation. The L current activated at −40 mV, and it reached a plateau at −20.1 ± 2.3 pA ( n = 6). Its activation time course was a single exponential with an activation time contant of 26.8 ± 2.3 ms ( n = 4). Current-voltage curves, kinetics, gating, response to BAY K 8644, nifedipine, amiloride, and different selectivity for Ba2+ and Ca2+ indicated that the underlying channels for the observed currents are only of the T- and L-types that resemble those of the endocrine secretory cells.