ATP increases [Ca2+]i and ion secretion via a basolateral P2Y-receptor in rat distal colonic mucosa.
Under resting conditions the mammalian distal colon is a NaCl-absorptive epithelium. NaCl absorption occurs at surface cells in colonic crypts. Intracellular Ca2+ or cAMP are important second messengers that activate NaCl secretion, a function that is most pronounced in crypt bases. In the present study we examined the effect of extracellular ATP on isolated crypts of rat distal colon using the fura-2 technique. Intracellular Ca2+ ([Ca2+]i) was measured spectrofluorimetrically either by photon counting or video imaging. ATP reversibly increased [Ca2+]i in crypt base cells with an EC50 of 4.5 micromol/l (n = 11). This [Ca2+]i increase was composed of an initial peak, reflecting intracellular store release, and a secondary plateau phase reflecting transmembrane influx. Digital video imaging revealed that agonist-induced [Ca2+]i elevations were most marked at the crypt base. In the middle part of the crypt ATP induced smaller increases of [Ca2+]i (peak and plateau) as compared to basal cells and in surface cells this [Ca2+]i transient was even further reduced. Attempts to identify the relevant P2-receptor demonstrated the following rank order of potency: 2MeS-ATP > ADP >/= ATP >> AMP > UTP > AMP-PCP > adenosine. In Ussing chamber experiments ATP (1 mmol/l) functioned as a secretagogue, increasing transepithelial voltage (Vte) and equivalent short-circuit current (Isc): Delta Isc = -36.4 +/- 5.4 microA/cm2, n = 17. Adenosine itself (1 mmol/l) induced an increase of Isc of similar magnitude to that induced by ATP: Delta Isc = -55. 1 +/- 8.4 microA/cm2, n = 9. The effect of adenosine, but not that of ATP, was fully inhibited by the A1/A2-receptor antagonist 8-(p-sulphophenyl)theophylline, 0.5 mmol/l, n = 4. Together these data indicate that: (1) basolateral ATP induces [Ca2+]i in isolated rat colonic crypts and acts as a secretagogue in the distal rat colon; (2) a basolateral P2Y-receptor is responsible for this ATP-induced NaCl secretion; (3) the ability of ATP to increase Isc in Ussing chamber experiments is not mediated via adenosine; and (4) the agonist-induced [Ca2+]i signals are mostly located in the crypt base, which is the secretory part of the colonic crypt.
Leipziger
J
J
Physiologisches Institut, Albert-Ludwigs-Universität, Hermann-Herder-Strasse 7, D-79104 Freiburg, Germany.
Kerstan
D
D
Nitschke
R
R
Greger
R
R
D000255:Adenosine Triphosphate / Q000494:pharmacology*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003106:Colon / Q000187:drug effects* / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D005260:Female; D007413:Intestinal Mucosa / Q000187:drug effects* / Q000378:metabolism; D017136:Ion Transport / Q000187:drug effects; D008297:Male; D051381:Rats; D018048:Receptors, Purinergic P2 / Q000187:drug effects*
D018048:Receptors, Purinergic P2; D000255:Adenosine Triphosphate; D002118:Calcium