Active force development by vascular smooth muscle in response to elevation of luminal pressure, or stretch, is termed as the myogenic response, which is independent of neural, metabolic, hormonal, and endothelial factors. Mechanosensitive ion channels, especially nonselective cation channels (NSC) or chloride channels have been considered as possible candidates for transducing mechanical events into the contractile response of the cell. We recorded single-channel currents from enzymatically dispersed rabbit pulmonary (PASMCs) and coronary arterial smooth muscle cells (CASMCs) (obtained from humanely killed animals) using the patch clamp technique. With 140 mM CsCl in the pipette solution, application of negative pressures through the pipette induced the activation of channels. The current-voltage relationship was linear in symmetrical ionic conditions, and the single channel conductances for Cs⁺, K⁺ and Na⁺ were 30, 36 and 27 pS, respectively. When NMDG⁺ was substituted for Cs⁺ in the pipette solution, inward currents were abolished whereas outward currents remained active, indicating that the channels were nonselective to cations. The same kind of channels were also observed in mesenteric arterial myocytes. We also investigated the distribution of mechanosensitive cation channels (NSC_MS) in myocytes from pulmonary arteries of various diameters. The open probabilities of NSC_MS (NP_O) at 50 mV activated by -10 cmH₂O negative pressure were 0.25 ± 0.18 (n = 7) and 0.71 ± 0.23 (n = 14) in large-diameter (≧ 1.5mm, o.d.) and small-diameter (≦ 600 μm, o.d.) pulmonary arteries, respectively. The chance of recording the NSC_MS was higher in small pulmonary arteries than in larger ones (90 vs. 20%). Moreover, the mean numbers of NSC_MS in the patch area were 4.45 ± 1.04 and 1.86 ± 0.69 in small and larger pulmonary arteries, respectively. Interestingly, the size-dependent distribution and activation of NSC_MS were not observed in coronary arteries. Considering the vaso-regulatory role of ATP released during hypoxic stimuli, we investigated whether the purinergic stimulation affects the activity of NSC_MS. When ATP,UTP (1 μM) was included in the pipette for the cell-attached configuration, the NP_O and the chance of recording the NSC_MS was largely increased PASMCs while not in CASMCs. However, the bath application of ATP or UTP in the cell-attached configuration had no significant effect on the NSC_MS, suggesting a membrane-delimited regulation mechanism. In summary, we firstly characterized the NSC_MS in various arterial myocytes, which could provide a principal mechanism of the myogenic tone generation. In addition, the size-dependent distribution of NSC_MS and the positive regulation by purinoceptors (P2Y) suggest that these channels might play a specific role in the regulation of pulmonary circulation, e.g. hypoxic pulmonary vasoconstriction.