OPRK1 and obesity due to melanocortin 4 receptor deficiency: Other GPCRs involved in neurohormonal signalling and associated with obesity and insulin resistance, such as OPRK1 [18] and ADRB2 [19], and receptors for peptide hormones, including somatostatin (SSTR1/2/5) and secretin (SCTR) (Fig. 2a, d), were also enriched in K cells.