Ghrelin decreases pancreatic islet blood flow, and GHRP-6 reverses this effect in fasted rats
Background
The peptide ghrelin, primarily produced in the stomach, also originates from pancreatic ε-cells during starvation. It is the endogenous ligand for the growth hormone secretagogue receptor (GHS-R1α) and is known for stimulating appetite and weight gain. However, its complete metabolic role, particularly any vascular functions within the pancreas, is not fully understood. It's known to act on islet endocrine cells, but whether it influences the delivery of hormones like insulin by modulating local blood flow is a key unanswered question. This study investigates if ghrelin directly affects pancreatic and islet blood flow, potentially revealing a new mechanism for regulating insulin release.
Study Design
Researchers conducted an animal study using rats. Ghrelin and the GHS-R1α receptor antagonist GHRP-6 were administered via intravenous injection. Blood flow in the pancreas and islets was measured using a microsphere technique. The study compared the effects in both fed and fasted rats to understand the influence of metabolic state. They also performed in vitro experiments on isolated islets from both fed and fasted rats to assess glucose-stimulated insulin release. Finally, they used immunohistochemistry to identify the location of GHS-R1α receptors within the pancreatic islets.
Results
Ghrelin administration decreased islet blood flow. Conversely, the antagonist GHRP-6 selectively increased islet blood flow in fasted rats, but not in fed rats. GHS-R1α receptors were identified not just on glucagon-producing cells but also appeared to be present in the islet arterioles, suggesting a direct vascular target. In fasted rats, GHRP-6 also improved the peak insulin response to glucose in vivo, which substantially blunted hyperglycemia. Furthermore, GHRP-6 doubled glucose-stimulated insulin release in vitro from islets obtained from both fed and fasted rats.
In fasted rats, the
GHS-R1αantagonist GHRP-6 caused a fourfold increase in islet blood flow.
Key Findings
- In fasted rats, GHRP-6 increased islet blood flow fourfold.
- GHRP-6 doubled glucose-stimulated insulin release in vitro from islets of both fed and fasted rats.
- Ghrelin administration decreased islet blood flow in rats.
- GHRP-6 improved the peak insulin response to glucose in vivo in fasted rats.
GHS-R1αreceptors were identified in islet arterioles, suggesting a direct vascular site of action.
Why It Matters
This research reveals a novel physiological mechanism where ghrelin acts as a local vasoconstrictor in pancreatic islets during fasting. This action restricts blood flow, thereby limiting the insulin response to hyperglycemia, likely as a glucose-sparing adaptation. For those exploring ghrelin-modulating peptides, this suggests that timing and nutritional status are critical; using a GHS-R1α antagonist like GHRP-6 during a fasted state could enhance insulin sensitivity and response by increasing blood flow to the islets. This vascular mode of action complements ghrelin's known direct effects on endocrine cells. It implies that manipulating the ghrelin system can influence not just hormone secretion but also hormone delivery, a new therapeutic angle for metabolic control.
