Regulation of capillary exchange capacity in postnatal swine intestine

To determine whether intrinsic regulation of capillary exchange capacity is age-dependent during early postnatal life, we measured the capillary filtration coefficient (Kf,c), an index of capillary exchange capacity, within isolated perfused intestine from 3- and 35-day-old swine at different arterial pressures. Kf,c was determined gravimetrically, by means of the flow-equilibration technique. During baseline perfusion at age-appropriate pressures, Kf,c averaged 0.203 +/- 0.020 vs. 0.361 +/- 0.040 ml.min-1 x mmHg-1 x 100 g in 35- vs. 3-day-old intestine. In older animals, a progressive rise in Kf,c was noted as arterial pressure was lowered: Kf,c increased 14, 35, and 98% above baseline when pressure was reduced 20, 40, and 60% below baseline, respectively. An inverse linear correlation between Kf,c and blood flow was demonstrated in this group. Kf,c also increased in 3-day-old intestine during progressive hypotension, but these changes were of less magnitude: Kf,c averaged 19, 17, and 23% above baseline when pressure was reduced 20, 40, and 60% below baseline, respectively. In contrast to the circumstance noted in older animals, the relationship between Kf,c and blood flow was not significant within 3-day-old intestine. Intraarterial infusion of 1 microgram/min of isoproterenol, an agent known to increase capillary exchange capacity, caused Kf,c to increase 80% within 3-day-old intestine group. We conclude that Kf,c increases to a greater extent in 35- than 3-day-old swine intestine in response to arterial pressure reduction. However, intestine from younger animals is capable of increasing capillary exchange capacity in response to stimuli other than arterial hypotension, as evidenced by its response to isoproterenol.