Transcapillary transport during secretion by the rabbit submandibular salivary gland

Fluid exchange in the rabbit submandibular gland has been studied with a view to improving understanding of the mechanisms underlying transcapillary transport during salivation. Using maximally vasodilated glands perfused in vitro, we found that acetylcholine had no significant effect on either hydraulic conductance (filtration coefficient, Lp) or reflection coefficient to albumin (sigma alb) of the gland microvessels. In vivo, parasympathetic nerve stimulation increased blood flow 20-fold, interstitial fluid production (total fluid efflux - saliva flow) increased approximately 10-fold, while filtration fraction remained constant. At the same time, isogravimetric capillary pressure increased about six-fold. There are several possible explanations for these findings, and it is argued that they are likely to include capillary recruitment. Thus, during salivation, parasympathetic nerve stimulation does not lead to a change in capillary permeability as such, but rather produces a relaxation of resistance vessels and a fall in precapillary resistance, accompanied by an increase in available capillary surface area. This leads to a rise in capillary blood flow and pressure, which in turn increase fluid filtration. These changes are self-limiting due to the resulting dilution of interstitial protein and rise in interstitial pressure, but during salivation, since the secretion is pumped out of the interstitium, this fluid transport has no net effect on transcapillary gradients.