Local regulation of oviductal blood flow

1. Blood flow to the oviduct is implicated in the genesis and maintenance of oviductal fluid, in this way contributing to the creation of an adequate medium for ovum/embryo physiology. Therefore, factors controlling the tone of the vessels supplying the oviduct would be expected to affect its luminal environment. In addition, cyclic changes in oviductal blood flow have been suggested to have mechanical functions in the transport of the ovum/embryo. 2. The vascular supply to the oviduct has a prominent adrenergic vasomotor control. A dense adrenergic innervation, together with the presence of a predominant population of alpha(1)-adrenoceptors, provides a contractile regulatory mechanism of oviductal blood flow. No evidence is available on the presence of beta-adrenoceptors. The scanty cholinergic innervation of mammalian oviduct is mainly confined to the vessels, where acetylcholine (ACh) has a vasodilatory effect by releasing endothelium-derived relaxing factors. 3. The presence of nerves containing neuropeptides has been shown in the oviduct. Specifically, a high density of neuropeptide Y- and vasointestinal peptide-containing nerve fibers has been found in relation to blood vessels, but their role in the neutral control of the oviduct blood flow remains to be established. To date, it is not known whether or not oviductal blood vessels receive perivascular nitrergic nerves. 4. Relaxing and contracting factors derived from endothelium also seem to have a modulatory role on oviductal vascular tone. Neurotransmitters or autacoids, such as ACh and histamine, acting on endothelial receptors, release nitric oxide (NO), which relaxes oviductal arteries through guanylyl cyclase activation and accumulation of cyclic GMP. In addition, the release of an endothelium-derived hyperpolarizing factor (EDHF), distinct from NO, by ACh has been shown in oviductal arteries. It acts through the opening of low-conductance Ca(2+)-activated K+ channels leading to hyperpolarization and relaxation. Furthermore, potent and long-lasting contractions induced by the endothelium-derived contractile factor, endothelin (ET), points to its role in the long-term regulation of oviductal vascular tone. 5. A particularly high density of 5-hydroxytryptamine (5-HT) and histamine, present in mast cells clustered in the vicinity of blood vessels, has been described in the oviduct. It is known that histamine elicits a relaxation of oviductal arteries that is partially endothelium-dependent and mediated by the activation of H1-receptors. The implication of histamine in both the increase in blood flow and edema around ovulation, as well as the existence of a functional antagonism between histamine and 5-HT in the regulation of oviductal blood flow, await further investigation. 6. Other factors, such as relaxing and contracting cyclooxygenase-derived products, may also participate in the modulation of blood flow to the oviduct. 7. An overall endocrine regulation of the oviductal vascular supply exists, acting by both direct effects on smooth muscle and modulation of neural and autocrine factors. This control enables cyclic changes in blood flow to the oviduct that are tightly coupled to the reproductive functions of the tube.