β‐adrenergic activation of Cl secretion and K secretion in guinea pig distal colonic mucosa proceeds via separate receptor complexes

Epinephrine (epi) stimulated ion secretion in isolated mucosa from guinea pig distal colon, measured as short‐circuit current. Epi responses included transient Cl (0~5 min) and sustained K secretion (sec). ICI‐118551 (β2‐selective antagonist) inhibited Cl sec (Kd = 2.7 nM), even though inhibition of K sec required combined β1/β2 antagonism. Co‐immunoprecipitation of β1 and β2 adrenergic receptors (βAdrR) provided support for this combined action. Both β1AdrR and β2AdrR also co‐IPed with arrestin‐2, suggesting an alternate signaling path. Whereas dideoxyadenosine (ddAdo; inhibits transmembrane adenylyl cyclase, tmAC) inhibited only Cl sec, KH7 (inhibits soluble adenylyl cyclase, sAC) inhibited only K sec. Consistent with bicarbonate dependence for sAC, sustained K sec was 42% smaller in HEPES buffered bathing media (CO2/HCO3 free), without altering transient Cl sec. Immunoblot for sAC indicated a presence of 54 and 74 kDa bands, consistent with sAC isoforms found in other tissues. ddAdo suppressed the biphasic epi activation of cAMP levels; KH7 left the early transient cAMP unaltered but lowered sustained levels; early cAMP transients remained in the absence of bicarbonate with sustained levels lowered. Thus, β2AdrR activation produced Cl sec via a transient cAMP elevation from tmAC, while β1AdrR/β2AdrR activation produced K sec via a sustained cAMP from sAC that was bicarbonate dependent. [NIH DK65845]