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 Ca(2+) 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(1) and A(3) adenosine receptor subtypes, the secondary messenger was the cytoplasmic Ca(2+) concentration ([Ca(2+)](cyt)). Accordingly, the A(1) or A(3)-mediated increase in [Ca(2+)](cyt) and in ureagenic activity were both inhibited by chelating Ca(2+) with either EGTA or BAPTA-AM. Also, Gd(3+) blocked both the increase in [Ca(2+)](cyt) and ureagenesis, suggesting that a Ca(2+) channel may be involved in the response to both A(1) and A(3). A partial effect was observed with the sarcoplasmic reticulum Ca(2+)-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, Gd(3+) 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 [Ca(2+)](cyt), while the message from the A(2A) and A(2B) adenosine receptor subtypes is relayed by [cAMP].
Department of Biochemistry, School of Medicine, National Autonomous University of México, Apdo Postal 70-159, México D.F., 04510, Mexico.
Raquel
R
Guinzberg
Salvador
S
Uribe
Antonio
A
Díaz-Cruz
Arturo
A
Hernández Cruz
Enrique
E
Piña
D000241:Adenosine / Q000378:metabolism; D000067956:Adenylyl Cyclase Inhibitors; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002614:Chelating Agents / Q000494:pharmacology; D000242:Cyclic AMP / Q000378:metabolism; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D022781:Hepatocytes / Q000187:drug effects / Q000378:metabolism*; D007097:Imines / Q000494:pharmacology; D008297:Male; D058915:Purinergic P1 Receptor Antagonists; D051381:Rats; D017208:Rats, Wistar; D018047:Receptors, Purinergic P1 / Q000378:metabolism*; D015290:Second Messenger Systems / Q000187:drug effects / Q000502:physiology*; D014508:Urea / Q000378:metabolism*
D000067956:Adenylyl Cyclase Inhibitors; D002121:Calcium Channel Blockers; D002614:Chelating Agents; D004791:Enzyme Inhibitors; D007097:Imines; D058915:Purinergic P1 Receptor Antagonists; D018047:Receptors, Purinergic P1; C070379:1,2-bis(2-aminophenoxy)ethane N,N,N',N'-tetraacetic acid acetoxymethyl ester; D004533:Egtazic Acid; C014925:RMI 12330A; D014508:Urea; D000242:Cyclic AMP; D000241:Adenosine; D002118:Calcium