A ~ S 7" a A C T This paper reports a description of methods for determining the diffusional permeability to potassium ions of single capillaries in the frog mesentery. By means of micropipettes, injections or infusions were delivered into a single capillary. The subsequent concentration variations in and about the capillary were followed with K+-sensitive microelectrodes. A theoretical analysis is provided which gives a quantitative frame of reference for evaluating the observed timeconcentration curves in terms of capillary permeability. The advantage of single capillary studies is that the surface area through which diffusion occurs is known as is the concentration difference across the capillary membrane. Three different techniques are: (a) the "single injection" method which represents an application of the indicator diffusion technique where a high-K + bolus is injected into a single capillary; (b) the "sack" method which determines the rate of K + disappearance from within and immediately outside an occluded capillary segment, after a brief increase in intracapillary K + concentration; and (c) the "interstitial diffusion" method which records time and spatial distribution of K + in the interstitial space after a step-change in intracapillary K + concentration. The methods gave an average potassium permeability of the capillary membrane of 67 x 10 -s cm s -I (SD: 23, n = 26) at room temperature.These figures are clearly higher than those previously reported in mammalian capillary studies using whole-organ techniques. In terms of the Pappenheimer pore model, this estimate of capillary permeability is consistent with the behavior of a membrane with a thickness of 1.0 t~m which possesses equivalent pores with a radius of 110 ~, a fractional pore area of 0.3%, and a pore density of 8 ~m -2.

Vascular sensitivity (VS) and reactivity (VR) of hindquarters, totally isolated from rats made diabetic with 45 mg/kg of streptozotocin (STZ), were determined at 1, 2, 4, 8, and 12 wk post-STZ. Age-matched controls received saline injections and were followed for comparable periods. The hindquarters were perfused at constant flow (8-10 ml/min) with a Tyrode-perfluorocarbon (FC-43)albumin-alpha-globulin solution gassed with 95% O2-5% CO2. Tissues were continuously weighed and venous pressure adjusted to maintain an isogravimetric condition. VS and VR were determined from norepinephrine (NE) dose-response curves generated by infusing stock NE (1 mg/ml) at progressively increasing rates (0.004-0.025 ml/min) into a constant tissue perfusion rate (8-10 ml/min) for sufficient time to reach a plateau (2-3 min). Delivered doses ranged from 0.4 to 2.5 micrograms/ml. VR was established as the perfusion pressure reached in response to 2.5 micrograms/ml NE (Pmax). VS was defined as the delivered NE dose that increased perfusion pressure to 50% of Pmax (ED50). VS increased slightly but significantly (P less than 0.01) by 1 wk post-STZ and remained above control throughout the 12-wk post-STZ period. VR also increased significantly (P less than 0.05) by 1 wk post-STZ and remained above control throughout the 12-wk post-STZ period.

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The circulating and tissue hematocrits of normal unanesthetized mice were determined by means of independent red cell and plasma volume measurements. The red cell volume-indicator which was used in this study was radioiron (Fe59) tagged red cells. The plasma volume data were derived by means of radioiodine (I131) labeled serum albumin and were reported earlier (Friedman, Proc. Soc. Exper. Biol. & Med. 88: 323, 1955). The hematocrits of the various tissues were found to be: for spleen 51.3, lung 47.9, muscle 49.9, liver 38.9, intestine, 32.2, skin 29.2 and kidney 24.0%. The total body hematocrit was 35.4% as compared to 48.4 for venous blood. All tissues, with the exception of spleen and lung, contained hematocrits which were lower than that of venous blood suggesting the presence of some mechanism within the various tissues which is capable of effectively separating plasma from red cells.