The isolated in vitro perfused spiral modiolar artery: pressure dependence of vasoconstriction

We developed a new technique, the isolated in vitro perfused spiral modiolar artery, which allowed the continuous measurement of the vascular diameter and control of the intravascular pressure. An isolated section of the spiral modiolar artery from the gerbil was perfused on one end with a set of concentric pipettes and occluded on the other end in order to apply a defined intravascular pressure in the range between 10 and 230 cm H2O. The preparation was continuously superfused with a NaCl solution. The diameter of the spiral modiolar artery in NaCl solution displayed little dependence on the applied intravascular pressure. The diameter was 73 +/- 10 microm (n = 5) at 10 cm H2O and increased with pressure to 85 +/- 7 microm (n = 5) at the highest applied pressure (220 or 230 cm H2O). Elevation of the K+ concentration from 3.6 to 150 mM in the superfusate caused a transient vasoconstriction. The amplitude of the K+-induced vasoconstriction depended strongly on the applied intravascular pressure. At 10 cm H2O the amplitude was maximal and the outer diameter decreased transiently by 49 +/- 9% (from 73 +/- 10 to 38 +/- 9 microm; n = 5). The amplitude of K -induced vasoconstriction was nearly maximal at pressures lower than 30 cm H2O, declined at higher pressures, and was not significantly different from zero at pressures larger than 100 cm H2O. These observations in conjunction with an estimation of the intravascular pressures in vivo suggest that cochlear blood flow can be regulated on two levels: (1) cochlear blood flow can be regulated by controlling the vascular diameter of the spiral modiolar artery (intracochlear blood flow regulation) and (2) intracochlear blood flow regulation can be modulated by altering the perfusion pressure which is controlled by the vasculature upstream of the cochlea.