Trp-His, a vasorelaxant di-peptide, can inhibit extracellular Ca ²⁺ entry to rat vascular smooth muscle cells through blockade of dihydropyridine-like l-type Ca²⁺ channels

Our previous findings regarding the biological activities of small peptides revealed that a di-peptide, Trp-His (WH), could play a role in the prevention of vascular lesions, including cell proliferation and atherosclerosis. Its vasoprotective effects could be associated with suppression of the vasocontraction signaling cascade, but the underlying mechanism(s) remains obscure. In this study, we attempted to elucidate the vasoprotective mechanism of WH, in opposing the proliferation of rat vascular smooth muscle cells (VSMCs). In VSMCs from 8 week-old male Wistar rat thoracic aortae, WH evoked a significant dose-dependent anti-proliferation effect, without cytotoxicity. In mitogen-stimulated cell experiments, 300 μM WH inhibited cytosolic Ca ²⁺ elevation in VSMCs induced by 10 μM angiotensin II (Ang II). Furthermore, WH suppressed extracellular Ca²⁺ entry into CaCl ₂-stimulated VSMCs. The biological capacity of WH as an intracellular Ca²⁺ ([Ca²⁺]_i) suppressor was also proven when 50 μM Bay K8644 was used to enhance Ca²⁺ entry via a voltage-dependent l-type Ca²⁺ channel (VDCC) and 300 μM WH elicited a 23% reduction in [Ca²⁺]_i. The absence of a reduction of the [Ca²⁺]_i by the mixture of tryptophan and histidine revealed the importance of the peptide backbone in the [Ca ²⁺]_i reduction effect. Furthermore, the WH-induced [Ca²⁺]_i reduction was abolished by verapamil, but not by nifedipine, indicating that WH likely binds to an extracellular site of the VDCC at a site similar to that of the dihydropyridine type-Ca²⁺ channel blockers.