Mechanism of 5-hydroxytryptamine receptor 4 mediated vasorelaxation in the isolated bovine lateral saphenous vein

Authors: TROTTA, RONALD - University Of Kentucky; Klotz, James

Submitted to: Journal of Federation of American Societies for Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2026
Publication Date: 2/4/2026
Citation: Trotta, R.J., Klotz, J.L. 2026. Mechanism of 5-hydroxytryptamine receptor 4 mediated vasorelaxation in the isolated bovine lateral saphenous vein. Journal of Federation of American Societies for Experimental Biology. 40:(7). https://doi.org/10.1096/fj.202504690R.
DOI: https://doi.org/10.1096/fj.202504690R

Interpretive Summary: Serotonin is a neurotransmitter or signaling molecule. It is involved in a number of different biological functions that range from mood, memory, digestion, blood clotting, and the regulation of the diameter of blood vessels (vasoconstriction or vasodilation). Serotonin is related to a potential problem with blood flow in grazing livestock called fescue toxicosis. Livestock that graze tall fescue grass can be exposed to toxins called ergot alkaloids that are produced by a fungus that lives in the plant. Ergot alkaloids can bind with serotonin receptors and disrupt their normal function. Our laboratory has characterized the effects that ergot alkaloids and serotonin have on constriction and more recently relaxation of blood vessels. There are 14 different serotonin receptors and in bovine leg veins relaxation is primarily caused by the stimulation of serotonin receptor subtype 4. This study sought to understand how this serotonin receptor can stimulate vasorelaxation by looking at a number of different paths for this response. Several associated signaling mechanisms were identified as responsible for vasorelaxation when this receptor was stimulated. The findings reported in this manuscript will be primarily of interest to other researchers looking to better understand how ergot alkaloids cause the negative effects seen in livestock that consume them. This work will ultimately benefit livestock producers as it has the potential to lead to an eventual treatment of fescue toxicosis.

Technical Abstract: To better understand the mechanisms of 5-hydroxytryptamine receptor 4 (HTR4)-mediated vasorelaxation, lateral saphenous veins from cattle (n = 4 to 7) were collected and assessed for vasoactivity in response to increasing concentrations of a selective HTR4 agonist (BIMU 8) in the absence and presence of inhibitors selective for downstream proteins involved with cyclic nucleotide-mediated signaling. Vessels were pre-contracted with 1x10-4 M phenylephrine and exposed to increasing concentrations of the HTR4 agonist. Vasoactive response data were normalized as a percentage of the maximum contractile response induced by the phenylephrine pre-contraction. Antagonism of HTR4 attenuated the vasorelaxation induced by BIMU 8. Removal of the endothelium did not influence HTR4 mRNA or protein expression or HTR4-mediated vasorelaxation. In the presence of inhibitors selective for adenylate cyclase or guanylate cyclase, 66% and 87% of the maximal HTR4-mediated vasorelaxation response was attenuated. In the presence of inhibitors selective for protein kinase A or protein kinase G, 66% and 98% of the maximal HTR4-mediated vasorelaxation response was attenuated. In the presence of non-selective blockers of Ca2+ and K+ channels, 65% and 64% of the maximal HTR4-mediated vasorelaxation response was attenuated. In the presence of inhibitors selective for myosin light chain phosphatase and small heat shock protein phosphorylation, 72% and 57% of the maximal HTR4-mediated vasorelaxation response was attenuated. HTR4-mediated vasorelaxation occurs through an endothelium-independent, cyclic nucleotide-dependent mechanism. Downstream proteins involved in the signaling cascade leading to vasorelaxation include protein kinase A and G, Ca2+ channels, K+ channels, myosin light chain phosphatase, and small heat shock proteins.