Cytosolic calcium (Cai) responses to arginine vasopressin (AVP) and angiotensin-II (Ang II) were examined in single rat adrenal zona glomerulosa (ZG) cells by monitoring fura-2 fluorescence with microspectrofluorimetry. ZG cells displayed dose-dependent Cai responses to a wide range of AVP and Ang II concentrations, starting from a threshold of 1 nM for AVP and less than 5 pM for Ang II. A dose-dependent delay of the onset of the Ca; response was observed with both hormones. The response delay for Ang II was consistently briefer than that for the same concentration of AVP, showing a 2-3 log unit separation in the dose-response relations. After the delay, cells typically responded with an abrupt increase in Cai(which peaked within 15 sec. The amplitude of the peak Ca; rise showed little dependency on AVP or Ang II concentration. At most AVP concentrations, the response consisted of Caioscillations, with apparent fusion of these Cajoscillations at the highest AVP concentrations (1-0.1 μM). Similar oscillatory behavior was found with stimulation by much lower Ang II concentrations (0.5 nM to 5 pM). There appeared to be a 2-3 log unit shift in the sensitivity toward AVP and Ang II when Ca; responses were compared. Sixty percent of ZG cells were responsive to AVP, while more than 90% displayed an elevation of Cajwith Ang II. The Cai and steroid responses to 100 nM AVP and 100 pM Ang II were compared, since these two doses are reported to stimulate the phosphoinositide system to a similar extent. Individual ZG cells tested with both hormones responded with equivalent peak Caichanges, but a slightly longer response delay for AVP. The mean Cairesponse and aldosterone production for each secretagogue displayed parallel kinetics during 30-min stimulations. After initial oscillations, the Caj response returned to control values within 15 min of 100 nM AVP application. Likewise, the steroid output was transient. In contrast, 100 pM Ang II produced maintained Ca; oscillations as well as a sustained and substantially greater aldosterone production for the same period of application. In conclusion, the disparate steroidogenic effects of AVP and Ang II appear to result from distinctly different Cajresponses elicited during maintained secretagogue stimulation.
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