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

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: A1, A2A, A2B and A3. Each receptor subtype was stimulated with a specific agonist while blocking all other receptor subtypes with a battery of specific antagonists. For the A1 and A3 adenosine receptor subtypes, the secondary messenger was the cytoplasmic Ca2+ concentration ([Ca2+]cyt). Accordingly, the A1 or A3-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 A1 and A3. 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 A2A or the A2B adenosine receptor subtypes, while it decreased slightly in response to stimulation of either A1 or A3. The stimulation of either the A2A or A2B 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 A2A and A2B, but not the response to either A1 or A3. Our results indicate that in the ureagenic liver response to adenosine, the secondary messenger for both, the A1 and A3 adenosine receptor subtypes is [Ca2+]cyt, while the message from the A2A and A2B adenosine receptor subtypes is relayed by [cAMP].