Location for a-BKCa in the presence and absence of the b1-subunit. Interestingly, and in contrast, the presence of the a-subunit in the expression system resulted in an increased proportion of b1-subunit being located at the cell surface. In earlier studies, Jackson and Blair suggested that BKCa is ‘silent’ in cremaster muscle vasculature under basal conditions, but may be ‘recruited’ under stimulated conditions. Such stimulation was suggested to include vasoconstriction evoked by catecholamines and high tissue PO2 levels. Whether such recruitment involves differences in splice variant expression, translocation of channel subunit proteins to the plasma membrane or post-translational modifications such as phosphorylation has not been fully elucidated. Given the high levels of a-subunit protein found at the membrane in both vessel types, it is unlikely that a simple difference in membrane vs. cytosolic pools explains the differences observed between cremaster and cerebral vessels. This does not, however, exclude the possibility that a dynamic alteration in channel protein trafficking occurs under other conditions. In the present study, we also found a very low level of expression of the SS4 a-subunit variant relative to the ZERO variant in midcerebral, with no detectable expression in cremaster arteries. Although expression of the SS4 variant in vasculature has been previously reported in cerebral and coronary arteries, its functional importance, particularly in native tissues such as small arteries, is unknown. Similarly, the functional significance of a lack of SS4 variant expression is unclear. Using a oocytes expression system, previous studies have suggested that ZERO and SS4 variants exhibit identical BKCa channel characteristics, including single-channel conductance and voltage dependent activation. Apart from differences in splice variant expression, it would be expected that the marked difference in total BKCa channel protein expression would be of functional significance. This is despite the large conductance in VSMCs from both vascular beds. Specifically, an,20-fold higher level of a-BKCa protein was detected in cerebral arteries compared with cremaster arteries. Importantly, this would be reflected at the plasma membrane because a similar proportion of total BKCa was surface located in both cerebral and cremaster arteries, as shown by our biotinylation assay. While measurements were performed on homogenates of whole vessels, the majority of signal would be expected to derive from the VSMC layers. Endothelial cells of healthy arteries are thought to be devoid of BKCa channels, although this point has been somewhat controversial. Cellular capacitance measurements performed in our previous studies indicate that VSMC size in the two vessel types is similar, suggesting that functional effects of the expression difference would not be compensated by differences in size alone.