NOS Inhibition Enhances Myogenic Tone by Increasing Rho-Kinase Mediated Ca2+ Sensitivity in the Male but Not the Female Gerbil Spiral Modiolar Artery

Abstract

Cochlear blood flow regulation is important to prevent hearing loss caused by ischemia and oxidative stress. Cochlear blood supply is provided by the spiral modiolar artery (SMA). The myogenic tone of the SMA is enhanced by the nitric oxide synthase (NOS) blocker L-NG-Nitro-Arginine (LNNA) in males, but not in females. Here, we investigated whether this gender difference is based on differences in the cytosolic Ca2+ concentration and/or the Ca2+ sensitivity of the myofilaments. Vascular diameter, myogenic tone, cytosolic Ca2+, and Ca2+ sensitivity were evaluated in pressurized SMA segments isolated from male and female gerbils using laser-scanning microscopy and microfluorometry. The gender difference of the LNNA-induced tone was compared, in the same vessel segments, to tone induced by 150 mM K+ and endothelin-1, neither of which showed an apparent gender-difference. Interestingly, LNNA-induced tone in male SMAs was observed in protocols that included changes in intramural pressure, but not when the intramural pressure was held constant. LNNA in male SMAs did not increase the global Ca2+ concentration in smooth muscle cells but increased the Ca2+ sensitivity. This increase in the Ca2+ sensitivity was abolished in the presence of the guanylyl cyclase inhibitor ODQ or by extrinsic application of either the nitric oxide (NO)-donor DEA-NONOate or the cGMP analog 8-pCPT-cGMP. The rho-kinase blocker Y27632 decreased the basal Ca2+ sensitivity and abolished the LNNA-induced increase in Ca2+ sensitivity in male SMAs. Neither LNNA nor Y27632 changed the Ca2+ sensitivity in female SMAs. The data suggest that the gender difference in LNNA-induced tone is based on a gender difference in the regulation of rho-kinase mediated Ca2+ sensitivity. Rho-kinase and NO thus emerge as critical factors in the regulation of cochlear blood flow. The larger role of NO-dependent mechanisms in male SMAs predicts greater restrictions on cochlear blood flow under conditions of impaired endothelial cell function.

Renal Autoregulation in Health and Disease

“…Both NO and ROS in VSMCs regulated RyR-mediated Ca 2ϩ mobilization (384, 1645), but by distinct signaling pathways (752,861,866,1224). NO dilated gracilis muscle arteries and afferent arterioles by activating MLCP (129, 1499,1503), and attenuated myogenic tone of nonrenal vessels by inhibiting Rho kinase in VSMCs (475,1214,1458). This may also occur in the renal microcirculation (1358).…”

Pressure-induced oxidative activation of PKG enables vasoregulation by Ca 2+ sparks and BK channels

“…Stimulation of BK channel activity by the nitric oxide (NO)/soluble guanylate cyclase (sGC)/cGMP-dependent protein kinase (PKG) pathway, which is important in mediating the effects of endothelial-dependent vasodilators, is not thought to be involved in modulating the myogenic response of small resistance arteries during basal tone because there is no evidence for pressure-induced release of NO from the endothelium4–7. Under physiological conditions, PKG exists as a dimer and oxidants such H 2 O 2 promote the formation of a disulfide bond between appositioned cysteine residues (Cys 42 ) within PKG dimers8.…”

Ryanodine-induced vasoconstriction of the gerbil spiral modiolar artery depends on the Ca2+ sensitivity but not on Ca2+ sparks or BK channels

“…Such changes in Ca 2+ sensitivity further augment the Ca 2+ -dependent myogenic vasoconstrictions [36–40]. We have previously shown that myogenic tone in the male, but not female, gerbil SMA is regulated not by changes in the SMC global Ca 2+ but by rho-kinase-mediated changes in the Ca 2+ sensitivity of contraction, which was revealed under inhibition of NO-mediated signaling [23]. The present study shows that the regulation of vascular tone in the female gerbil SMA is also determined by the Ca 2+ sensitivity of the myofilament, with the crucial difference that increase in intravascular pressure significantly lowered the Ca 2+ sensitivity (Fig.…”

“…Data points for concentration curves were obtained by averaging diameter and fluorescence intensity measurements over the last 30s of each Ca 2+ step and normalizing against the average value obtained in PSS at 60 cmH 2 O. Data points from individual vessels were fitted to a modified Hill equation:where Dia is the normalized diameter, Max is the diameter at 60 cmH 2 O in PSS containing 1 mM Ca 2+ , Base is the maximum achievable constriction with respect to Max for the female gerbil SMA estimated from previous observations [23], [Ca 2+ ] is the normalized cytosolic Ca 2+ concentration, h is the slope coefficient, and FC 50 is the fold-change in the global cytosolic Ca 2+ concentration that is necessary for a half-maximal constriction. The slope coefficient h was set to -5.2 and Max was clamped to 100 %.…”