GCG and diabetes mellitus: To date, there are several diabetes drugs that indirectly act on glucagon secretion in various ways: the administration of GLP-1 reduces glucagon secretion in patients with T2D [51], and metformin treatment, which improves hepatic insulin resistance, suppresses glucagon secretion [52]; nevertheless, the direct effects of these treatments on glucagon secretion and α-cell function remain to be elucidated.