A peptide of the phylloseptin family from the skin of the frog Hylomantis lemur (Phyllomedusinae) with potent in vitro and in vivo insulin-releasing activity

A peptide with the ability to release insulin from the rat BRIN-BD11 clonal β cell line was isolated from norepinephrine-stimulated skin secretions of the Lemur leaf frog Hylomantis lemur Boulenger,1882. Determination of the primary structure (FLSLIPHVISALSSL.NH₂) demonstrated that the peptide belongs to the phylloseptin family whose members have previously been identified in other Phyllomedusinae species. A synthetic replicate of the peptide, termed phylloseptin-L2, produced a significant stimulation of insulin release (134% of basal rate, P < 0.01) from BRIN-BD11 cells at a concentration of 30 nM, with a maximum response (301% of basal rate, P < 0.001) at a concentration of 3 μM. Phylloseptin-L2 did not stimulate release of the cytosolic enzyme, lactate dehydrogenase at concentrations up to 3 μM, indicating that the integrity of the plasma membrane had been preserved. The stimulatory action was maintained in the absence of extracellular Ca²⁺ and in the presence of verapamil (50 μM) and diazoxide (300 μM) suggesting that mechanism of action of the peptide did not primarily involve influx of Ca²⁺ or closure of ATP-sensitive K⁺ channels. Administration of phylloseptin-L2 (50 nmol/kg body weight) into mice significantly (P < 0.05) increased total release of insulin and improved glucose tolerance during the 60 min period following an intraperitoneal injection of glucose (18 mmol/kg body weight). It is concluded that the peptide shows potential for development into a therapeutically valuable agent for the treatment of Type 2 diabetes.