**Core Concept**
Gastrin is a peptide hormone that stimulates gastric acid secretion by the parietal cells of the stomach. It plays a crucial role in regulating the digestive process, particularly in the context of meal ingestion. Gastrin's release is tightly regulated by various factors, including endoluminal peptides in the stomach.

**Why the Correct Answer is Right**
The correct answer (D) highlights the key mechanism of gastrin release. Gastrin is secreted by G cells in the antrum of the stomach, which are stimulated by endoluminal peptides, such as gastrin-releasing peptide (GRP) and cholecystokinin (CCK). These peptides bind to their respective receptors on G cells, triggering a signaling cascade that leads to gastrin release. This response is essential for the physiological regulation of gastric acid secretion in response to food intake. The endoluminal peptides in the stomach serve as the primary stimuli for gastrin release, ensuring that gastric acid secretion is coordinated with meal ingestion.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastrin is not secreted by parietal cells of the stomach. Instead, parietal cells are responsible for secreting gastric acid (hydrochloric acid) in response to gastrin stimulation.

**Option B:** Gastrin is not secreted by alpha cells of the pancreas. Alpha cells in the pancreas secrete glucagon, a hormone that raises blood glucose levels by stimulating glycogenolysis and gluconeogenesis.

**Option C:** Gastrin does not decrease bicarbonate content in pancreatic secretion. In fact, gastrin stimulates the pancreas to secrete bicarbonate-rich fluid, which helps to neutralize gastric acid in the duodenum.

**Clinical Pearl / High-Yield Fact**
A key clinical application of understanding gastrin regulation is in the diagnosis and management of Zollinger-Ellison syndrome, a rare disorder characterized by excessive gastrin secretion leading to severe peptic ulcer disease.

**✓ Correct Answer: D. It gets stimulated by endoluminal peptides in stomach**