Vasospasm in rat middle cerebral arteries: evidence for modulation by T-type calcium channels and IKCa channels

Francesc Jimenez-Altayo1, Christopher Garland1, Alister McNeish2. 1Oxford Unversity, Oxford, United Kingdom, 2Bath University, Bath, United Kingdom.

In rat middle cerebral artery following inhibition of NO synthase (NOS), constriction similar to vasospasm develops that is associated oscillations in membrane potential (Em) and tone in a process that appears to be mediated by L-type voltage gated cation channels (VGCCs; Yuill et al. 2009). However, several studies in different vascular beds suggest that T-type voltage-gated Ca2+ channels may be involved in maintenance of vascular tone (Navarro-Gonzalez et al., 2008, Morita et al., 2002). Therefore we investigated if similar mechanisms contribute vascular tone in middle cerebral arteries.

Rat middle cerebral arteries were mounted in a wire myograph. The effect of: reducing external [Ca2+] (from 2.5 mM to 0.25 mM), endothelium denudation and/or blocking: voltage dependent Na+ (Tetrodotoxin), T-type VDCC (Mibefradil), calcium activated potassium channels (KCa; TRAM-34 and apamin ), on the oscillations generated in the presence of L-NAME, indomethacin and iberiotoxin were assessed as simultaneous changes in tension and smooth muscle cell Em using glass microelectrodes.

Inhibition of NO synthase (NOS) and BKCa evoked sustained constriction (5.7±0.2 mN, n=41; P<0.05) often termed vasospasm this was associated with depolarization (Em -35.7±1.1 mV, n=40; P<0.05) and development of rapid oscillations in Em and tension that were temporally coupled (frequency of 0.84±0.02 Hz and 0.80±0.05 Hz, respectively; amplitude of 22.6±1.3 mV and 0.19±0.02 mN, respectively, n=41), and were not affected by blockade of voltage dependent Na+ channels, or endothelium denudation. Oscillations in Em were abolished by T-type Ca2+ channel blockade (frequency of 0.09± 0.05 Hz, amplitude of 0.04±0.02 mN, n=5; P<0.05) and attenuated by reduction of extracellular Ca2+, both treatments were associated with relaxation (31.4 ± 3.7% and 69.5 ± 8.5%, n=5, respectively). Inhibition of IKCa also attenuated oscillations in Em (frequency of 0.66 ± 0.10Hz, amplitude of 8.70 ± 3.10 mV, n=5) and tension (frequency of 0.44 ± 0.11 Hz, amplitude of 0.15 ± 0.03 mN, n=5)and evoked a small relaxation (15.7 ± 4.0 %, n=4). The effect of TRAM-34 was not modified by endothelium denudation or inhibition of SKCa.

Our data reveal novel roles for T-type calcium and IKCa channels in regulation of the depolarizing oscillations in membrane potential that are essential in the development of vasoconstriction and vasomotion associated with a model of vasospastic conditions in rat middle cerebral arteries.

Morita et al. (2002) Br J Pharmacol 137, 467-476.
Navarro-Gonzalez et al. (2009) Clin. Exp. Pharmacol. Physiol. 36, 55-66.
Yuill et al (2009) J .Vasc. Res. In press.

Supported by the British Heart Foundation.