Abstract 5837: Prolonged in vivo Exposure to Nitroglycerin Enhances the Role of BKCa in Nitroglycerin-Induced Relaxation by Increasing BKCa Expression and Current Density
The therapeutic efficacy of nitroglycerin (NTG) is limited by the development of tolerance. Many studies have examined potential causes of nitrate tolerance, but little is known about possible adaptive processes that may occur in nitrate-tolerant blood vessels. We hypothesized that prolonged in vivo exposure to NTG induces a compensatory response involving BKCa channels in NTG-induced vasorelaxation. Rats were treated with either placebo or NTG (0.6 mg/hr) patches for 3 days. In isolated aortic rings contracted with norepinephrine (0.1 μM), tolerance was evident by ~7-fold rightward shift in the NTG concentration-response curve in rings from rats treated with NTG patches (pD2=6.6 ± 0.3) vs placebo (pD2=7.4 ± 0.1; n=6). Iberiotoxin (100 nM), a selective BKCa-blocker, had no effect on NTG-induced relaxation of rings from control rats, but caused a further 7-fold rightward shift in the response to NTG in rings from tolerant rats (pD2 =5.7±0.3). Glyburide (1 μM), apamin (1 μM), and 4-aminopyridine (1 mM) had no effect on NTG responses in control or tolerant rings. Immunoblot analysis indicated a 2.5-fold increase in expression of BKCa α- and β1-subunits in arteries from tolerant rats as compared with control (n=6). Expression of Kv1.2 channel proteins was similar in control and tolerant arteries. In whole cell voltage-clamp studies, NTG (0.1 – 10 μM) caused a concentration-dependent increase in BKCa current density in aortic smooth muscle cells from tolerant rats, but had no effect on BKCa current in myocytes from control animals. Maximal BKCa current density was increased by NTG (10 μM) in tolerant aortic myocytes at +80 mV (from 22.6 ± 3.0 to 52.5 ± 5.5 pA/pF; n=12). Similarly, NS1619 (3 – 30 μM), a selective BKCa activator that binds to the α-subunit, and tamoxifen (1 – 10 μM), a BKCa activator that binds to the β1-subunit, caused a greater increase in BKCa current in tolerant than in control cells (n=6). We conclude that there is both a quantitative and qualitative difference in NTG-induced vascular relaxation in nitrate tolerance. Relaxation is independent of BKCa in control arteries, but prolonged in vivo exposure to NTG selectively increases expression of BKCa channel proteins, resulting in a major role for BKCa in NTG-induced relaxation of nitrate tolerant arteries.
This research has received full or partial funding support from the American Heart Association, Midwest Affiliate (Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Nebraska, North Dakota, South Dakota & Wisconsin).