In rat hepatocytes, different adenosine receptor subtypes use different secondary messengers to increase the rate of ureagenesis

摘要

In rat hepatocytes, the role of cAMP and Ca2+ as secondary messengers in the ureagenic response to stimulation of specific adenosine receptor subtypes was explored. Analyzed receptor subtypes were: A(1), A(2A), A(2B) and A(3). Each receptor subtype was stimulated with a specific agonist while blocking all other receptor subtypes with a battery of specific antagonists. For the A, and A3 adenosine receptor subtypes, the secondary messenger was the cytoplasmic Ca2+ concentration ([Ca2+](cyt)). Accordingly, the A(1) or A(3)-mediated increase in [Ca2+](cyt) and in ureagenic activity were both inhibited by chelating Ca2+ with either EGTA or BAPTA-AM. Also, Gd3+ blocked both the increase in [Ca2+](cyt) and ureagenesis, suggesting that a Ca2+ channel may be involved in the response to both A(1) and A(3). A partial effect was observed with the sarcoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin. The concentration of cyclic AMP ([cAMP]) increased in response to stimulation of either the A(2A) or the A(2B) adenosine receptor subtypes, while it decreased slightly in response to stimulation of either A(1) or A(3). The stimulation of either the A(2A) or A(2B) adenosine receptor subtypes resulted in an increase in [cAMP] and an ureagenic response which were not sensitive to EGTA, BAPTA-AM, Gd3+ or to thapsigargin. In addition, the adenylyl cyclase inhibitor MDL12,330A blocked the ureagenic response to A(2A) and A(2B), but not the response to either A(1) or A(3). Our results indicate that in the ureagenic liver response to adenosine, the secondary messenger for both, the A(1) and A(3) adenosine receptor subtypes is [Ca2+](cyt), while the message from the A(2A) and A(2B) adenosine receptor subtypes is relayed by [cAMP]. (c) 2006 Elsevier Inc. All rights reserved.