The renal proximal tubule (PT) is a major site for a complete tissue renin-angiotensin system (RAS) and produces endogenous angiotensin II (ANG II). The present studies demonstrate autocrine RAS feedback in a line of origin-defective SV40 plasmid transformed immortalized rat PT cells (IRPTC) designated as line 93-p-2-1, which are highly differentiated and express all RAS components. Receptor competition assays and Southern blot following RT-PCR demonstrated that these IRPTC express AT1 and AT2 angiotensin receptor subtypes. Autocrine RAS feedback was examined following exposure to ANG II (10(-8) M), and it was noted that angiotensinogen mRNA increases significantly by 1 h and remains elevated through 24 h. The AT1 blocker losartan prevents this increase. Moreover, ANG II upregulates expression of ANG II receptor mRNA (both AT1 and AT2). Thus the present studies demonstrate positive ANG II feedback with angiotensinogen and ANG II receptors in PTC, suggesting that the main site of such intrarenal feedback in vivo is within PT. ANG II secreted by line 93-p-2-1 is increased by isoproterenol, suggesting beta-adrenergic regulation in IRPTC.