EXPRESSION OF TYPE-1 ANGIOTENSIN-II RECEPTOR SUBTYPES AND ANGIOTENSIN-II-INDUCED CALCIUM MOBILIZATION ALONG THE RAT NEPHRON

The localization of two type 1 angiotensin II receptor subtype mRNA, A T(1A) and AT(1B), was determined by reverse transcription-PCR on micro dissected glomeruli and nephron segments. The coupling sensitivity of these two receptor sun-types was evaluated by measuring variations in intracellular calcium ([Ca2+](i)) elicited by angiotensin II (Ang II) in structures expressing either AT(1A) or AT(1B) mRNA, using Fura-2 fl uorescence. The highest expression of AT(1) mRNA was found in glomerul us, proximal tubule, and thick ascending limb. In glomerulus, AT(1A) a nd AT(1B) mRNA were similarly expressed, whereas in all nephron segmen ts AT(1A) mRNA expression was dominant (approximately 84%). The increa se in [Ca2+](i) elicited by 10(-7) mol/L Ang II was highest in proxima l segments (Delta [Ca2+](i) is approximately equivalent to 300 to 400 nmol/L) and thick ascending limb (Delta [Ca2+](i) is approximately equ ivalent to 200 nmol/L). In glomerulus and collecting duct, the respons e was lower (Delta < 100 nmol/L). The median effective concentrations for Ang II were of the same order of magnitude in glomerulus (12.2 nmo l/L), in which both AT(1A) and AT(1B) are expressed, and in cortical t hick ascending limb (10.3 nmol/L), in which AT(1A) is almost exclusive ly expressed. The Ang II-induced calcium responses were totally abolis hed by the AT(1) receptor antagonist losartan (1 mu mol/L) but not by the AT(2) antagonist PD 123319 (1 mu mol/L). In the absence of externa l Ca2+, the peak phase of the response induced by 10(-7) mol/L Ang II was reduced and shortened, suggesting that a part of the [Ca2+](i) inc rease originated from the mobilization of the intracellular Ca2+ pool. In conclusion, these results demonstrate that in the rat kidney: (I) AT(1A) is the predominant AT(1) receptor subtype expressed in the neph ron segments, (2) glomerulus is the only structure with a relatively h igh AT(1B) mRNA content, and (3) AT(1A) and AT(1B) receptor subtypes d o not differ in their efficiency for the activation of calcium second- messenger system.