CA2-DEPENDENCE AND NIFEDIPINE-SENSITIVITY OF VASCULAR TONE AND CONTRACTILITY IN THE ISOLATED SUPERFUSED SPIRAL MODIOLAR ARTERY IN-VITRO()

The regulation of the vascular diameter of the spiral modiolar artery may play a major role in the regulation of cochlear blood flow and tis sue oxygenation since the spiral modiolar artery provides the main blo od supply to the cochlea. The goal of the present study was to determi ne whether vascular lone and contractility of the spiral modiolar arte ry depend on the presence of extracellular Ca2+ and involves nifedipin e-sensitive Ca2+ channels. The spiral modiolar artery was isolated and superfused in vitro and the diameter was measured continuously by vid eo microscopy. Isolated segments of the spiral modiolar artery had an outer diameter of 61 +/- 3 mu m (n = 59) and displayed vasomotion char acterized by 5-15 clearly distinguishable constrictions per min. Remov al of Ca-2+ from the superfusion medium caused a reversible relaxation and cessation of vasomotion and was used to determine the magnitude o f basal vascular tone. The basal vascular tone consisted of a sustaine d reduction of the vascular diameter to 95.1 +/- 0.3% (n = 51) of the maximal diameter in Ca2+-free medium. Nifedipine reduced the basal vas cular tone with an IC50 Of (1.1 +/- 0.3) x 10(-9) M although 22% of th e basal vascular tone was insensitive to nifedipine. Elevation of the K+ concentration from 3.6 to 150 mM caused a transient vasoconstrictio n which was dependent on the presence of extracellular Ca2+. Nifedipin e fully inhibited K+-induced vasoconstriction with an IC50 of (2.0 +/- 0.7) x 10(-9) M. Norepinephrine (10(-4) M) caused a transient vasocon striction and an increase of vasomotion at branch points of the spiral modiolar artery. Norepinephrine-induced vasoconstriction was fully in hibited in the absence of Ca2+ and partially inhibited by 10(-7) M nif edipine. These observations suggest that the spiral modiolar artery co ntains voltage-dependent nifedipine-sensitive Ca2+ channels which are involved in the maintenance of basal vascular tone as well as in the m ediation of K+- and norepinephrine-induced contractility. Further, the data suggest that cytosolic Ca2+ stores, if present in the spiral mod iolar artery, are of limited capacity compared to other vessels. (C) 1 998 Published by Elsevier Science B.V.