Chronic nicotine alters NO signaling of Ca2+ channels in cerebral arterioles

Abstract

Smoking is a major health hazard with proven deleterious effects on the cerebral circulation, including a decrease in cerebral blood flow and a high risk for stroke. To elucidate cellular mechanisms for the vasoconstrictive and pathological effects of nicotine, we used a nystatin-perforated patch-clamp technique to study Ca2+ channels and Ca2+-activated K+ (BK) channels in smooth muscle cells isolated from cerebral lenticulostriate arterioles of rats chronically exposed to nicotine (4.5 mg/kg per day of nicotine free base, 15 to 22 days via osmotic minipump). Two major effects were observed in cells from nicotine-treated animals compared with controls. First, Ca2+ channels were upregulated (0.48±0.03 pS/pF [20 cells] versus 0.35±0.01 pS/pF [31 cells], P<0.005) and BK channels were downregulated (12±3 pA/pF [14 cells] versus 34±7 pA/pF [14 cells], P<0.05), mimicking the effect of an apparent decrease in bioavailability of endogenous NO. Second, normal downregulation of Ca2+ channels by exogenous NO (sodium nitroprusside [SNP], 100 nmol/L) and cGMP (8-bromo-cGMP, 0.1 mmol/L) was absent, whereas normal upregulation of BK channels by these agents was preserved, suggesting block of NO signaling downstream of cGMP-dependent protein kinase. In pial window preparations, chronic nicotine blunted NO-induced vasodilation of pial vessels and the increase in cortical blood flow measured by laser-Doppler flowmetry, demonstrating the importance of Ca2+ channel downregulation in NO-induced vasorelaxation. These findings elucidate a new pathophysiological mechanism involving altered Ca2+ homeostasis in cerebral arterioles that may predispose to stroke.