In the present study, we tested whether the &agr;1Asubunit, which encodes a neuronal isoform of voltage-dependent Ca2+channels (VDCCs) (P-/Q-type), was present and functional in vascular smooth muscle and renal resistance vessels. By reverse transcription–polymerase chain reaction and Southern blotting analysis, mRNA encoding the &agr;1Asubunit was detected in microdissected rat preglomerular vessels and vasa recta, in cultures of rat preglomerular vascular smooth muscle cells (VSMCs), and in cultured rat mesangial cells. With immunoblots, &agr;1Asubunit protein was demonstrated in rat aorta, brain, aortic smooth muscle cells (A7r5), VSMCs, and mesangial cells. Immunolabeling with an anti-&agr;1Aantibody was positive in acid-macerated, microdissected preglomerular vessels and in A7r5 cells. Patch-clamp experiments on aortic A7r5 cells showed 22±4% (n=6) inhibition of inward Ca2+current by &ohgr;-Agatoxin IVA (10–8mol/L), which in this concentration is a specific inhibitor of P-type VDCCs. Measurements of intracellular Ca2+in afferent arterioles with fluorescence-imaging microscopy showed 32±9% (n=10) inhibition of the K+-induced rise in Ca2+in the presence of 10–8mol/L &ohgr;-Agatoxin IVA. In microperfused rabbit afferent arterioles, &ohgr;-Agatoxin IVA inhibited depolarization-mediated contraction with an EC50of 10–17mol/L and complete blockade at 10–14mol/L. We conclude that the &agr;1Asubunit is expressed in VSMCs from renal preglomerular resistance vessels and aorta, as well as mesangial cells, and that P-type VDCCs contribute to Ca2+influx in aortic and renal VSMCs and are involved in depolarization-mediated contraction in renal afferent arterioles.