Functional evidence for the presence of TASK-1 in the rat mesenteric artery M.J. Gardener, I.T. Johnson, G. Edwards & A.H. Weston. School of Biological Sciences, University of Manchester, G.38 Stopford Building, Oxford Road, Manchester, M13 9PT, UK.	Print Abstract Search PubMed for: Gardener MJ Johnson IT Edwards G Weston AH

Two-pore potassium channel subunits contain four membrane-spanning domains with two pore-forming regions. The subunits are thought to dimerize in order to create the functional channel with the four pore-forming regions combining to form the channel pore (Lesage et al., 2000). To date twelve functional channels are known to exist and each is classified into a family group according to its electrophysiological and pharmacological characteristics. We have previously demonstrated the presence of several two-pore potassium channels in the rat mesenteric artery (Gardener et al., 2002) using both RT-PCR and immunohistochemistry. In the present study we demonstrate functional evidence for the presence of such channels in resistance vessels from the rat.

Male Sprague-Dawley rats (150-200g) were killed by stunning followed by cervical dislocation and third order mesenteric arteries were dissected free in ice-cold Tyrode's solution (pH 7.4). Intact vessels were pinned to a Sylgard base and superfused (10ml.min^-1) at 37^oC with HEPES-buffered Tyrode's solution (pH 7.4) gassed with 100% O₂. Solutions were made more acidic (pH 6.4) or alkaline (pH 8.4) by the addition of HCl or NaOH respectively. Vessels were impaled from the adventitial side using micro-electrodes filled with 3M KCl (resistance 40-80M).

Resting membrane potential at pH 7.4 (E_M -49.9±0.5mV) was significantly depolarised by switching to a more acidic Tyrode's solution (pH 6.4:- E_M-43.2±0.6mV, P<0.05, paired t-test n=5).

A significant hyperpolarisation was also seen with a shift to an alkaline pH (pH 8.4:- E_M -57.5±1.0mV, P<0.05, n=5) when compared to the resting membrane potential at pH 7.4. Addition of 10µM anandamide completely and reversibly abolished the effects of switching to pH 8.4 and returned the membrane potential to a level comparable to that seen with Tyrode's solution at pH 6.4 (pH 8.4 plus anandamide:- E_M -44.5±0.6mV, P<0.05, n=5).

Members of the TASK (TWIK-related Acid Sensitive K⁺ channel) family are known to be modulated by changes in extracellular pH, with a shift towards more acidic pH causing the channels to close whilst increased channel opening is seen with shifts to more alkaline pH. Predicted H⁺ dose-response curves indicate TASK-1, TASK-2 and TASK-3 channels are modulated by changes in pH between 8.4 and 6.4 (O'Connell et al., 2002), although mRNA for TASK-3 was not detected by RT-PCR in the rat mesenteric artery. Anandamide blocks TASK-1 channels at low concentrations whilst TASK-2 channels are unaffected (Maingret et al., 2001).

These data are consistent with the presence of functional TASK-1 channels in the mesenteric artery. We propose that shifts to an acidic extracellular pH cause TASK-1 channels to close and lead to vessel depolarisation whilst a shift to an alkaline extracellular pH leads to channel opening and hyperpolarisation of the vessels. This increase in membrane potential can be reversed by application of anandamide and subsequent block of TASK-1.

Gardener M.J. et al. (2002) Br. J. Pharmacol. Proc. Supp. 135, 306P.
Lesage F. et al. (2000) Am. J. Physiol. 279, F793-801.
Maingret F. et al. (2001) EMBO J. 20, 47-54.
O'Connell A.D. et al. (2002) Biochim. Biophys. Acta. 1566, 152-61.