Abstract 616: Diminished K Channel Inhibition Impairs Serotonin-Mediated Vasoconstriction in the Middle Cerebral Arteries From the Fawn Hooded Hypertensive Rat

Abstract

Fawn Hooded Hypertensive (FHH) rat exhibit impaired pressure-mediated myogenic response (MR) in the middle cerebral arteries (MCAs) that is associated with an increase in large conductance potassium (BK) channel function. Introgression of 2.4 Mbp region of BN (Brown Norway) chromosome 1 into FHH rats (FHH.1BN) restored MR and BK channel function. The present study assessed the hypothesis that FHH rats also exhibit impaired serotonin (5-HT)-mediated vasoconstriction due to diminished BK channel inhibition compared to FHH.1BN rats. We used pressure myography and patch-clamp to measure vessel response and K channel function respectively and Fluo 4 method to measure cytosolic calcium. Basal myogenic tone of MCAs as measured by change in diameter from 22oC to 37oC temperature was ~2.6 fold lower in FHH rats compared to FHH.1BN rats (FHH, 8.1 ± 2% and FHH.1BN, 21.7 ± 3%, n = 4; p<0.05). Vascular smooth muscle cells (VSMCs) of FHH rats have significantly more negative membrane potentials (Em) (–32 ± 1 mV; n=9) compared to FHH.1BN rats (-16 ± 2mV; n=10). 5-HT-mediated vasoconstriction was lower in MCAs isolated from FHH rats compared to FHH.1BN rats (5-HT: 1μM; FHH, 36 ± 5%; FHH.1BN rats, 58 ± 9%; n = 4; p<0.05). 5-HT-mediated BK channel inhibition was less in FHH rats (3μM 5-HT: FHH, 8 ± 3%; FHH.1BN, 39 ± 4%; n = 4; p<0.05). 5-HT did not affect VSMC membrane potential in FHH rats compared to FHH.1BN rats (delta change in Em: FHH, -2 ± 2mV; n = 4; FHH.1BN, -9 ± 1 mV; n = 5; p<0.05). 5-HT-mediated increase in calcium fluorescence (F/Fo) during plateau phase was blunted in the VSMCs isolated from FHH rats compared to FHH.1BN rats (FHH: 1 + 0.01% Vs 1.22 + 0.02% in FHH.1BN (3 rats)). Finally, inhibition of BK channel restored 5-HT-mediated vasoconstriction in MCAs of FHH rats but did not affect FHH.1BN vessels (5-HT + Paxilline; FHH, 2.6 ± 0.6 fold; FHH.1BN, 1.4 ± 0.4 fold; n = 5; p<0.05). In conclusion, mutation in the genes located in 2.4 Mbp region of FHH rats disrupts 5-HT-mediated BK channel inhibition that prevents the raise in [Ca2+]i and this may contribute to impaired 5HT-mediated vasoconstriction.