ATP is co-released with nor-adrenaline from sympathetic nerves and acts through P2X receptors on smooth muscle cells to mediate membrane depolarization and contraction. The P2X₁ subtype is the predominant P2X receptor found in vascular smooth muscle. In earlier studies we have shown novel spatially localized Ca²⁺ signals in the smooth muscle adjacent to perivascular nerves (Lamont & Wier, 2002; Lamont et al. 2003). We hypothesized that these novel Ca²⁺ transients represented Ca²⁺ entering smooth muscle cells through P2X₁ receptors activated by ATP released from sympathetic nerves and we therefore called them ‘junctional Ca²⁺ transients’ or jCaTs. Owing to the lack of potent and subtype-selective P2X-receptor antagonists the physiological role of P2X₁ receptors has been difficult to determine. Recently P2X₁ receptor-deficient mice have been developed (Mulryan et al. 2000; Vial & Evans, 2002). Using small pressurized mesenteric arteries from these mice we have examined the contractile responses to sympathetic nerve stimulation and the underlying Ca²⁺ signals. Mice were humanely killed by a lethal overdose of CO₂. Isolated arteries were mounted between two glass pipettes and the intra-luminal pressure controlled (30mmHg). Arteries were loaded with the Ca²⁺ indicator Fluo-4 AM. Brief pulses (< 0.2ms) of electrical field stimulation (EFS) preferentially stimulated the perivascular nerves. The neurogenic origin of all responses was confirmed by their abolition by TTX (1μM). In wild type (WT) arteries confocal microscopy of Fluo-4 fluorescence during EFS (just sub-threshold for contraction) revealed jCaTs in the smooth muscle cells adjacent to the perivascular nerves (18.63±1.96 Ca²⁺ transients were observed during 10s of 3Hz EFS in an area 75 by 50μm, 6 arteries, n=12, all data expressed as mean±S.E.M.). In P2X₁ null (KO) arteries the same stimulation protocol produced no jCaTs. Some Ca²⁺ transients were observed (0.7193±0.227, protocol as above); these were not jCaTs but Ca²⁺ sparks, as assessed by their resistance to TTX, temporal spatial characteristics and abolition by ryanodine. In addition in KO arteries no contraction was produced by αΒmethylene ATP (a potent P2X receptor agonist, 10μM). In the WT arteries the frequency of jCaTs increased with nerve stimulation frequency (10.77±2.42 events at 1 Hz, 27.90±7.90 events at 10 Hz, protocol as above). Application of α-latrotoxin (2.5nM), an agent which evokes exocytotic release of neurotransmitters from a variety of nerve terminals, produced a significant (t test, p<0.05) increase in the frequency of spontaneous events from 1.143±0.553 events/30s, to 9.909±0.553 events/30s in WT arteries (3 arteries, n=6). Only sparks were observed in the KO arteries (3.300±1.012 events/30s,) and there was no significant change in spark frequency in α-latrotoxin (3.142±1.933, 3 arteries, n=6). With stronger (levels producing contraction) EFS a biphasic contraction of the artery was observed in the WT arteries. In the P2X₁ null arteries, however, the initial rapid phase was absent; this phase has previously been attributed to be the purinergic component by pharmacological methods. The data support the hypothesis that jCaTs represent Ca²⁺ entering smooth muscle cells through P2X₁ receptors and that the initial rapid component of the neurogenic contraction in these small arteries is purinergic in origin and more specifically originates from P2X₁ receptors.