Acyl-Ghrelin Influences Pancreatic β-Cell Function by Interference with K_ATP Channels

The negative effect of AG on SSC was abolished in β-cells and isolated islets from SUR1-KO mice. A: Representative measurement of glucose-induced oscillations of [Ca²⁺]_c in β-cells of SUR1-KO mice in the presence of AG. AG had no significant effect. B: Summary of measurements of mean [Ca²⁺]_c with AG in β-cells of SUR1-KO mice in all experiments. C: Representative measurement of the effect of AG on V_m in the presence of 10 mmol/L glucose. AG slightly depolarized V_m. D: Summary of all experiments of V_m measurements with AG in β-cells of SUR1-KO mice. E: AG had no effect in GSIS in isolated islets of SUR1-KO mice at 10 mmol/L glucose. The numbers in the columns indicate the number of experiments with different β-cell clusters or isolated islets from at least three mice. **P ≤ 0.01; ***P ≤ 0.001.

AG modulated K_ATP channels indirectly. A: Representative measurement of K_ATP-channel activity in the inside-out patch configuration in β-cells of WT mice. ATP closed most K_ATP channels. The addition of AG slightly increased current density but did not open K_ATP channels. B: Summary of all experiments performed under these conditions with AG and ATP in β-cells of WT mice. C: Representative measurement of K_ATP current in the perforated-patch configuration with AG in β-cells of WT mice. In the presence of 6 mmol/L glucose, most K_ATP channels were closed and, therefore, the K_ATP current was low. After application of AG, the current increased. D: Summary of all experiments of this series with AG. The numbers in the columns indicate the number of experiments with different β-cell clusters from at least three mice. **P ≤ 0.01; ***P ≤ 0.001.

UAG interfered with the effect of AG on β-cells. A: Representative measurement of glucose-induced oscillations of [Ca²⁺]_c in β-cells with UAG and AG. UAG alone did not affect glucose-induced oscillations of [Ca²⁺]_c. In the presence of UAG, AG no longer decreased glucose-induced oscillations of [Ca²⁺]_c, demonstrating the counteracting potential of UAG. B: Summary of all measurements of mean [Ca²⁺]_c with UAG and AG. C: Representative measurement of the effect of UAG and AG on V_m in the presence of 10 mmol/L glucose. UAG suppressed the AG-induced effects on V_m and AP frequency. D: Summary of all experiments of the V_m series with UAG and AG. E: UAG tended to increase GSIS under steady-state conditions but did not alter GSIS significantly. F: In the presence of UAG, AG did not decrease GSIS in the presence of 10 mmol/L glucose. The numbers in the columns indicate the number of experiments with different β-cell clusters or isolated islets from at least three mice. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001.

The action of AG on SSC can be averted by a combination of an antagonist and an inverse agonist of the GHSR1a. A: Representative measurements of glucose-induced oscillations of [Ca²⁺]_c in the presence of [D-Lys³]-GHRP-6 and AG at 10 mmol/L glucose. The inhibition of glucose-induced oscillations of [Ca²⁺]_c occurred even in the presence of the GHSR1a antagonist (n = 10 of 21 measurements). The GHSR1a antagonist [D-Lys³]-GHRP-6 alone was not sufficient to inhibit the effect of AG in every measurement of glucose-induced oscillations of [Ca²⁺]_c. B: Summary of all experiments in this series with determination of [Ca²⁺]_c. C: Representative measurements of the effect of [D-Lys³]-GHRP-6 and AG on the V_m in the presence of 10 mmol/L glucose. [D-Lys³]-GHRP-6 itself did not affect the V_m and inhibited the effect of AG in 5 of 10 measurements. D: Summary of all experiments of the V_m measurements with [D-Lys³]-GHRP-6 and AG. E: Representative measurement of the effect of K-(D1-NaI)-FwLL-NH₂ as an inverse agonist of the GHSR1a, [D-Lys³]-GHRP-6 as an antagonist of the GHSR1a, and AG on the V_m at 10 mmol/L glucose. F: Summary of all experiments in this series of V_m measurements. G: AG decreased GSIS significantly at 10 mmol/L glucose. The addition of the GHSR1a antagonist [D-Lys³]-GHRP-6 could not completely avert the effect of AG. The combination of K-(D1-NaI)-FwLL-NH₂ and [D-Lys³]-GHRP-6 prevented the AG-evoked decrease of the GSIS at 10 mmol/L glucose. The numbers in the columns indicate the number of experiments with different β-cell clusters or isolated islets from at least three mice. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001.

The inverse agonist K-(D1-NaI)-FwLL-NH₂ affected the amplifying pathway and reversed the inhibitory effect of AG on GSIS. A: The inverse agonist K-(D1-NaI)-FwLL-NH₂ decreased GSIS in the presence of 10 mmol/L glucose. B: In the presence of 3 mmol/L glucose and 60 mmol/L K⁺, the inhibitory effect of K-(D1-NaI)-FwLL-NH₂ was completely blocked. The addition of IBMX showed that an alteration in GSIS was still possible under these conditions. C: The inhibitory effect of AG in the presence of 10 mmol/L glucose (columns 1 and 2 from the left) was reversed in the presence of K-(D1-NaI)-FwLL-NH₂ (columns 3 and 4). The numbers in the columns indicate the number of experiments with isolated islets from at least three mice. *P ≤ 0.05; **P ≤ 0.01; ***P ≤ 0.001.

The effect of AG was not mediated via SST; SSTR possibly is involved. A: Representative measurement showing the attenuation of glucose-induced oscillations of [Ca²⁺]_c in β-cells of SUR1-KO mice by SST. B: Summary of all experiments in this series of measurements of mean [Ca²⁺]_c with SST in β-cells of SUR1-KO mice. The effect of SST was separately calculated for the intervals 0–5 min and 5–20 min. C: AG did not diminish GSIS in the presence of the SSTR2–5 antagonist H6056 in whole islets of WT mice. The numbers in the columns indicate the number of experiments with different β-cell clusters or isolated islets from at least three mice. **P ≤ 0.01; ***P ≤ 0.001.