SOCs are a distinct class of Ca²⁺-permeable channel in smooth muscle cells of cerebral arterioles (Flemming et al. 2002). TRPC1 is involved in the function of these SOCs but other molecular elements are unknown (Xu & Beech, 2001). In this study we explored the sensitivity of the SOCs to a range of blockers.

Male rabbits were killed by an I.V. overdose of sodium pentobarbitone according to Schedule 1 procedures. Arteriolar fragments were isolated from pial membrane and loaded with fura-PE3. [Ca²⁺]_i was measured by ratiometric imaging from smooth muscle cells in arterioles. Effects of LOE908 were studied using fluo-4. Except when the effects of 60 mM K⁺ were studied, solutions contained 0.01 mM methoxyverapamil to block voltage-gated Ca²⁺ channels. Store depletion was induced using 1000 nM thapsigargin or 0.01 mM cyclopiazonic acid. Data are expressed as means ± S.E.M. The n values refer to the number of cells. The number of individual arterioles was at least three for each data set.

Store depletion induced sustained Ca²⁺ entry that depended on extracellular Ca²⁺. It was strongly inhibited by bath-applied 0.01 mM Gd³⁺ (97.2 ± 2.1 %, n = 24 cells), 0.01 mM La³⁺ (70.4 ± 8.7 %, n = 21) or 0.1 mM Ni²⁺ (56.8 ± 10.9 %, n = 19). Gd³⁺ and Ni²⁺ had no effect on [Ca²⁺]_i in the absence of store depletion (n = 43 and 23) and, although La³⁺ had an effect, the absolute size of this effect was small (24 % of that in store-depleted cells, n = 15). 2-Aminoethoxydiphenylborate (0.075 mM) inhibited SOC-mediated Ca²⁺ entry in 67 % of cells (66.5 ± 2.7 % inhibition, n = 20) with no effect in controls (n = 35). A 1 h pre-incubation with wortmannin (0.01 mM) strongly inhibited the Ca²⁺ re-entry signal in store-depleted cells (ΔR_340/380 0.017 ± 0.004, n = 20 vs. 0.093 ± 0.017, n = 18; P < 0.05, Student’s unpaired t test).

SOCs in smooth muscle cells of choroidal arterioles are blocked by nifedipine (Curtis & Scholfield, 2001). Nifedipine (1000 nM) abolished the [Ca²⁺]_i rise in response to 60 mM K⁺ (91.5 ± 4.1 %, n = 19) but had no effect on SOC-mediated Ca²⁺ entry (n = 24). SOC-mediated Ca²⁺ entry was slightly inhibited (< 15 %) or resistant to other inhibitors: 0.01 mM SKF96365 (n = 16); 0.01 mM LOE908 (n = 56); 0.1 mM ruthenium red (n = 10); 0.1 mM capsaicin (n = 10); 0.1 mM sulindac (n = 19); 0.5 mM streptomycin (n = 15); or 1:10 000 Grammostolla spatulata venom (n = 20).

The data are consistent with TRPC1 being a subunit of these SOCs. Resistance to ruthenium red suggests that TRPVs are not involved. These pial arteriolar SOCs are distinct from background channels and SOCs in choroidal arterioles.

We thank the BHF for support.

All procedures accord with current UK legislation.