Tachyphylaxis of the ECL-cell response to PACAP: receptor desensitization and/or depletion of secretory products

Abstract

Background and purpose: Rat stomach ECL cells secrete histamine and pancreastatin in response to gastrin and pituitary adenylate cyclase-activating peptide-27 (PACAP). This study applies microdialysis to explore how ECL cells in situ respond to PACAP and gastrin.

Experimental approach: Both peptides were administered by microinfusion into the gastric submucosa. The microdialysate was analysed for histamine and pancreastatin (ECL-cell markers) and for somatostatin (D-cell marker).

Key results: Microinfusion of PACAP (0.01-0.3 nmol microl(-1)) raised microdialysate histamine and pancreastatin dose-dependently. The response was powerful but short-lived. The response to gastrin was sustained at all doses tested. It is unlikely that the transient nature of the histamine response to PACAP reflects inadequate histamine synthesis, since the pancreastatin response to PACAP was short-lived too, and both gastrin and PACAP activated ECL-cell histidine decarboxylase. Unlike gastrin, PACAP mobilized somatostatin. Co-infusion of somatostatin abolished the histamine-mobilizing effect of PACAP. However, pretreatment with the somatostatin receptor type-2 antagonist (PRL-2903) did not prolong the histamine response to PACAP, suggesting that mobilization of somatostatin does not explain the transient nature of the response. Repeated administration of 0.1 nmol microl(-1) of PACAP (1 h infusions, 1 h intervals) failed to induce a second histamine response. Pretreatment with a low dose of PACAP (0.03 nmol microl(-1)) abolished the response to a subsequent near-maximal PACAP challenge (0.3 nmol microl(-1)).

Conclusion: The transient nature of the histamine response to PACAP reflects desensitization of the PACAP receptor and/or exhaustion of a specific storage compartment that responds to PACAP but not to gastrin.

Figure 1

Time course of enterochromaffin-like (ECL)-cell histamine (a) and pancreastatin (PST) (b) mobilization in response to increasing doses of pituitary adenylate cyclase-activating peptide (PACAP) administered via the microdialysis probes (local microinfusion; horizontal line). (c) Dose–response curves for the histamine- and PST-releasing effects of PACAP (integrated 3-h increments). (d) Histamine and PST mobilization in response to local microinfusion of a near-maximal dose of gastrin (0.1 nmol μl⁻¹) is shown for comparison. Means±s.e.means (vertical lines) are shown; n=4–9.

Figure 2

Histamine mobilization in response to microinfusion of pituitary adenylate cyclase-activating peptide (PACAP) (0.1 nmol μl⁻¹), or gastrin (0.1 nmol μl⁻¹) and of PACAP+gastrin. The combination of the two peptides seemed to induce an additive effect. Means±s.e.means (vertical lines) are shown; n=5–9.

Figure 3

Histidine decarboxylase (HDC) activity in homogenates of small specimens collected from the stomach wall along the microdialysis probe after local microinfusion of saline, pituitary adenylate cyclase-activating peptide (PACAP; 0.1 nmol μl⁻¹) or gastrin (0.1 nmol μl⁻¹) for 3 h. Means±s.e.means (vertical lines) are shown; n=5–6. ^**P<0.01, ^***P<0.001.

Figure 4

Histamine mobilization in response to local microinfusion of pituitary adenylate cyclase-activating peptide (PACAP) (horizontal line) in hypergastrinaemic rats (omeprazole pretreatment for 4 days) and hypogastrinaemic rats (food deprivation for 24 h). The high basal histamine concentrations in the microdialysate in the hypergastrinaemic rats can be ascribed to the high serum gastrin concentration (see text). Means±s.e.means (vertical lines) are shown; n=6.

Figure 5

Histamine mobilization in response to local microinfusion of gastrin or gastrin+α-fluoromethylhistidine (α-FMH), as indicated by the horizontal line. Means±s.e.means (vertical lines) are shown; n=4–6.

Figure 6

(a) Somatostatin (SST) mobilization in response to local microinfusion of pituitary adenylate cyclase-activating peptide (PACAP; horizontal bar). Gastrin was without effect; n=5–8. (b) Histamine mobilization (integrated 3-h response) following microinfusion of gastrin or PACAP, PACAP with SST, PACAP with the SST receptor type 2 (SSTR2) antagonist PRL-2903 (PRL) i.v. (1.5 mg kg⁻¹ bolus followed by 1.5 mg kg⁻¹ h⁻¹ for 3 h) or PACAP with SST and PRL; n=6–10. (c) Histamine mobilization in response to PACAP microinfusion (horizontal line) with or without the concomitant intravenous infusion of PRL; n=5, ^*P<0.05. In (a–c) means±s.e.means (vertical lines) are shown.

Figure 7

Effect of intravenous infusion of a near-maximally effective dose of gastrin on histamine mobilization in rats administered either pituitary adenylate cyclase-activating peptide (PACAP) or saline via microdialysis probes. Means±s.e.means (vertical lines) are shown; n=5–7.

Figure 8

(a) Histamine mobilization in response to repeated 1 h administrations of pituitary adenylate cyclase-activating peptide (PACAP; 0.1 nmol μl⁻¹, local microinfusion) given at 1 h intervals. n=6. Histamine (b) and pancreastatin (PST) (c) mobilization in response to microinfusion of a near-maximally effective dose of PACAP in rats pretreated with a low dose of PACAP (indicated by horizontal lines); n=5. In (a–c) means±s.e.means (vertical lines) are shown.