Predominant distribution of nifedipine-insensitive, high voltage-activatedCa2+ channels in the terminal mesenteric artery of guinea pig

We have found nifedipine-insensitive (NI), rapidly inactivating, voltage-de pendent Ca2+ channels (current, NI-I-Ca) with unique biophysical and pharma cological properties in the terminal branches of guinea pig mesenteric arte ry, by using a whole-cell mode of the patch-clamp technique. The fraction o f NI-I-Ca appeared to increase dramatically along the lower branches of mes enteric artery, amounting to almost 100% of global I-Ca in its periphery, W ith 5 mmol/L Ba2+ as the charge carrier, NI-I-Ca was activated with a thres hold of -50 mV, peaked at -10 mV, and was half-activated and inactivated at -11 and -52 mV, respectively, generating a potential range of constant act ivation near the resting membrane potential. The NI-I-Ca was rundown resist ant, was not subject to Ca2+-dependent inactivation, and exhibited the pore properties typical for high voltage-activated Ca2+ channels; Ba2+ is appro ximate to 2-fold more permeable than Ca2+, and Cd2+ is a better blocker tha n Ni2+ (IC50, 6 and 68 mu mol/L, respectively), Relatively specific blocker s for N- and P/Q-type Ca2+ channels such as omega-conotoxins GVIA and MVIIC (each 1 mu mol/L) and omega-agatoxin IVA (1 mu mol/L) were ineffective at inhibiting NI-I-Ca, whereas nimodipine partially (10 mu mol/L; approximate to 40%) and amiloride potently (approximate to 75% with 1 mmol/L; IC50; 107 mu mol/L) blocked the current. Although these properties are reminiscent o f R-type Ca2+ channels, expression of the alpha(1E) mRNA was not detected u sing reverse transcriptase-polymerase chain reaction. These results strongl y suggest the predominant presence of NI, high voltage-activated Ca2+ chann els with novel properties, which may be abundantly expressed in peripheral small arterioles and contribute to their tone regulation.