Determinants of terminal mesenteric artery resistance during the first postnatal month

Experiments were conducted to delineate the vascular effector systems that contribute to setting mesenteric vascular tone in swine during the first postnatal month. Terminal mesenteric arteries (TMA), which function as resistance vessels, were studied in vitro with a microvascular perfusion system allowing independent pressure and flow manipulation. When pressure was varied 0-100 mmHg in the absence of flow, TMA from 1-day-old animals demonstrated myogenic vasoconstriction, whereas TMA from 40-day-old animals did not. In 1- but not 40-day-old TMA, the endothelin A (ETA) receptor antagonist BQ-610 shifted the pressure-diameter curve upward, whereas the ETB receptor antagonist BQ-788 and the L-arginine analog N-G-monomethyl-L- arginine (L-NMMA) shifted the curve downward; in all instances, myogenic vasoconstriction was preserved. Flow eliminated myogenic vasoconstriction in 1- day-old TMA, i.e., diameter increased as a function of pressure. The effect of BQ-610 was lost under flow conditions; however, BQ-788 and N-acyl-L-Trp-3,5-bis-( trifluoromethyl) benzyl ester, an antagonist specific to the substance P neurokinin-1 (NK1) receptor, shifted the pressure-diameter curve downward in the presence of flow, whereas L-NMMA restored myogenic vasoconstriction. Adding flow had no effect on the pressure-diameter relationship in 40-day-old TMA. Other blocking agents, including prazosin, losartan, indomethacin, and charybdotoxin, had no effect on the pressure-diameter relationship in either age group under flow or no-flow conditions. Constitutive production of nitric oxide (NO) and endothelin-1 participates in setting resistance in 1- day-old TMA, and important stimulants to NO production include flow and activation of ETB and NK1 receptors. In contrast, 40-day-old TMA act as passive conduits in which the elastic properties of the vessel are the primary determinant of diameter.