Distensibility of capillaries in the bat wing.

Preliminary experiments in our laboratory have shown that the distensibility characteristics of the capillary compartment in the bat wing depended upon its location in the vascular tree. The capillaries were then divided into arteriolar, middle and venular segments (according to their proximity to precapillary sphincters or nonmuscular venules). The bat was enclosed in an airtight box, one wing protruding through a slit and extended over a glass plate for microscopic observations. Continuous recordings of the diameter of the capillary segments were obtained; after 5 min of control recordings, the box pressure was raised in steps of 25 mm Hg to a maximum of 100 mm Hg and then returned to control level. The duration of each step was 4 min. Each increase of the pressure led to the dilatation of the capillary, but its arteriolar segment appeared to be more distensible than the middle and venous ones. After shifting the box pressure, the diameter increase was gradual and capillary distensibility decreased with increasing pressure (the venular segment showed the most prominent reduction in distensibility). These findings suggest the existence of a longitudinal gradient of distensibility in the capillary compartment.

Cessation and onset of muscle capillary flow at simultaneously reduced perfusion and transmural pressure.

The effect of simultaneously reduced perfusion and transmural pressure on muscle capillary flow was investigated in the tensor plagiopatagii muscle of unanesthetized bats. Perfusion and transmural pressure were simultaneously changed by suction applied to a sealed box containing the body of the bat, while the wing with the muscle under observation outside the box was exposed to atmospheric pressure. Flow ceased in almost all of the muscle capillaries at box pressures ranging from -46 to -96 mmHg (n = 40; median value: -66 mmHg). When flow in the muscle had ceased, flow was still present at a reduced level in capillaries on top of the muscle and in the web. No signs of obstruction to flow in the feeding or collecting vessels were found. The results are not in accordance with the phenomena causing cessation of flow as predicted by the theories of critical closure or interfacial tension. To restore flow in the muscle capillaries suction to the box had to be reduced by 14 mmHg (medium value; range 4 to 35 mmHg). The latter data conforms to the possible existence of yield stress in small vessels.

Pressure regulation in muscle of unanesthetized bats.

In unanesthetized bats direct microvascular pressure measurements were made in the tensor plagiopatagii muscle without surgical intervention. Pressure recordings showed great variability due to vasomotion and other physiological stimuli, but were in the same order of magnitude as described in muscle of other species in the anesthetized state. A substantial part of the pressure gradient is dissipated at the level of the small arterioles. Arterial pressure was changed by depressurizing a box containing the body of the bat, while the muscle was under atmospheric condition. In all larger arterioles and in 12 of 19 smaller arterioles a linear relation existed between microvascular pressure and box pressure. Of 19 small arterioles, 7 exhibited a clear biphasic regulatory response to a decrease in arterial pressure. The data are discussed in relation to different models of regulation.

Interspecific comparison of subcutaneous tissue fluid pressure in the bat wing.

Tissue fluid pressures were recorded from subcutaneous tissues of three bat species by means of an improved servo-micropipet pressure recording system. Experimental animals were restrained but unanesthetized during the procedure by methods which avoided vascular occlusion. Tissue fluid pressures averaged +0.46 +/- 2.08 mm Hg (n = 12) in the Mexican free-tailed bat, +0.21 +/- 0.63 mm Hg (n = 14) in the pallid bat, and -0.31 +/- 1.01 mm Hg (n = 12) in the little brown bat. None of the average values differed significantly (P greater than 0.25) from the others or from atmospheric pressure (P greater than 0.20). Tissue fluid pressures in the subcutis of the bat were near atmospheric in agreement with the results obtained in the bat and other species by earlier investigators using both hypodermic needles and micropipets.

Postnatal changes in hematology of the bat Antrozous pallidus.

Blood oxygen capacity increases with growth as the result of increasing hemoglobin concentration, which is accompanied by increasing red blood cell count and hematocrit. Hematological profile of the postnatal bat approaches that of the adult by 42 days of age when the animal begins to fly. Hematological development in the bat is similar to that in other altricial small mammals.

Microvascular myogenic reaction in the wing of the intact unanesthetized bat.

Microvascular dimension and flow responses to stepwise changes in arterial and venous pressures, ranging from zero to +100 mmHg and zero to -75 mmHg have been recorded. Observations were made in arterioles, terminal arterioles, and precapillary sphincters in the wing web of intact, unanesthetized bats. The results show for all categories of vessels that with reduced transmural pressures there is a progressive increase in mean diameter and a decrease in rhythmic vasomotion rate. Flow changes are variable. For elevated transmural pressures there is a vasoconstriction with drastic flow reduction that is inconsistent with metabolic control. However, after prolonged elevation of pressure there is a progressive increase in flow, suggesting a "metabolic escape". Computed wall tension remains reasonably constant for a wide range of transmural pressures, suggesting that wall tension may be the controlled variable. These findings support the hypothesis of a myogenic reaction as a mechanism for maintenance of basal vascular tone in the intact unanesthetized bat.

Interstitial fluid oncotic pressures in rabbit subcutaneous tissue.

Interstitial fluid oncotic pressures in rabbit subcutaneous tissues were measured by chronically implanted membrane osmometers. Pairs of osmometers were used, one using a membrane permeable and one impermeable to plasma proteins. Measurements from the small-pore membranes averaged 10.2 mmHg while pressures measured from the large-pore membrane osmometers averaged -1.2 mmHg, indicating that the measured pressures were osmotic rather than hydrostatic in nature. These values are in agreement with previously published values from this laboratory on oncotic pressures of excised rabbit skin and with a computer-simulation study of capillary fluid balance. The oncotic pressures described in this study are of the s-me general magnitude as those of lymph from most organs in the body.

Osmotic interaction of plasma proteins with interstitial macromolecules.

The osmotic interaction of plasma proteins with collagen and hyaluronate has been evaluated by measuring the oncotic pressure of mixed solutions of varying composition. Collagen, despite its insolubility, exhibits a pronounced volume exclusion effect on plasma proteins, and the oncotic pressure of mixed solutions is considerably higher than that of the plasma protein stock solution. The volume exclusion of collagen on small molecules such as sucrose is negligible. A solution composed of 1.6% plasma proteins, 20% collagen, and .4% hyaluronate in Ringer solution, approximating the composition of the interstitium, was found to yield higher oncotic pressures than those previously reported from the interstitium. The probable role of impurities and degradation in the isolation process is discussed. Results reported earlier from in vitro and in vivo studies indicated that tissue oncotic pressures are considerably higher than generally recognized and that tissue fluid is in probable osmotic equilibrium with lymph in skin and muscle.

Ground substance mucopolysaccharides and plasma proteins: their role in capillary water balance.

The osmotic interaction of mucopolysaccharides and plasma proteins, normally present in the interstitium, has been investigated. It has been found that hyaluronate-plasma protein mixtures may be treated as a two-phase system and that the two phases are in osmotic equilibrium. The osmotic pressures exerted by these mixtures are higher than the algebraic sum of the two components. At concentrations normally present in the interstitium of skin and muscle (0.6% mucopolysaccharides and 2% protein), the osmotic pressure exerted by the mixture is on the order of 10 mmHg, which is in agreement with predictions from earlier computer-simulation studies. The partition of fluid between the gel-like mucopolysaccharide compartment and the free-fluid containing the protein is approximately 50% in the "gel" phase at concentrations found in the interstitium. The volume exclusion effects of the interstitial mucopolysaccharides are significant, both in terms of selection of tracer molecules for interstitial volume measurements and also as an osmotic buffering mechanism which aids in maintaining the partition of fluid between the circulation and the interstitial space.