The development of capillaries in the rat heart

In the adult rat, studies by others have shown that capillaries of the heart are lined by a continuous endothelial layer. Some other important mural features are : a distinct and continuous lamina densa of the basal lamina, uniformly thick lateral processes of endothelial cells, adhesive and discontinuously occluding junctions between apposing cells. In addition, coated vesicles and pits are rarely seen, while a high proportion of the endothelial luminal and abluminal surfaces, as well as the cytoplasm, is occupied by plasmalemmal vesicles. These vesicles are more frequently found in the lateral processes than in the perikaryon (Fig. 3). It was my purpose in the present study to examine those morphological features of developing heart capillaries known to be important in capillary function in the adult. The hearts of 16, 19 and 21 day fetal and of 1 and 2 day neonatal Sprague- Dawley rats were removed, diced and fixed by immersion in a dilute Karnovsky fixative and processed routinely for TEM. Several fetal hearts (16 and 19 day) were removed and processed similarly 60 seconds after carbon (Pelikan Ink) had been injected into the umbilical vein.

Permeability and model testing of heart capillaries by osmotic and optical methods

Rabbits were anesthetized with Nembutal; their hearts were removed and perfused with Ringers solution. Osmotic weight transients were then produced by test solute infusion into the perfusate. The rate constant for organ weight change was used to predict test solute venous concentration (Cv) and the distribution volume (V) during an osmotic transient by use of the Johnson and Wilson (6) model for capillary tissue exchange. By use of Cr EDTA, strongly purple in solution, we compared the above predictions with a value of Cv obtained directly by optical measurement of outflow venous concentration and values of V calculated from our measurements of organ weight and the known sucrose distribution volume. The close agreement between both sets of values with a permeability coefficient of 5 x 10-5 cm/s and a volume of distribution of 3.2 ml/10 g heart lead us to conclude that the model used closly represents the conditions of the isolated perfused heart, and that both the osmotic transient and the extraction measurements provide good estimates of organ capillary permeability.

Permeability of mammalian heart capillaries to sucrose and inulin

Rabbit hearts were perfused with modified Ringer's solution containing 10 g sucrose or inulin per liter. After various perfusion times, ventricular samples were analyzed for their content of the respective saccharide. The tissue content of sucrose rose to half its apparent maximum value in approximately 6 sec; the corresponding time for inulin was approximately 90 sec. We concluded that at the high perfusion rates employed (40 ml/min per heart) inulin permeation is limited by passage across the capillary membrane and is practically unaffected by perfusion rate and interstitial diffusion, while sucrose permeation is limited partly by passage across the capillary membrane and partly by perfusion rate. We calculated capillary permeability coefficients of 0.4 x 10–5 cm/sec for inulin and 11 x 10–5 cm/sec for sucrose.

An Estimate of Reflection Coefficients for Rabbit Heart Capillaries

Isolated perfused rabbit hearts have been used to determine the reflection coefficients, σ, of the heart capillaries to certain lipoid-insoluble substances. This was done 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.