Abstract Body: Transgender males (TMs; assigned female at birth) on testosterone gender-affirming hormone therapy (T-GAHT) may experience reduced protective effects of estrogen (E2) typically seen in pre-menopausal cisgender females (CFs). E2 stimulates endothelial receptors, activating endothelial nitric oxide synthase (eNOS) and nitric oxide (NO) release, a potent vasodilator. TMs initiating T-GAHT at younger ages may face higher risk of earlier onset cardiovascular disease (CVD). While research on GAHT and CVD has progressed, the effects of T-GAHT on TMs' microvasculature remain understudied. We hypothesized that resistance arterioles from TMs on T-GAHT exhibit endothelial dysfunction compared to CFs.

We collected human resistance arterioles from subcutaneous adipose tissue of healthy TMs, CFs, and cisgender males (CMs) to compare vascular function via endothelium-dependent vasoreactivity by videomicroscopy. Key findings: 1) TMs on T-GAHT (n=8) showed impaired vasodilation to E2 compared to CFs (n=11), similar to levels of CMs(n=4)(p<0.0001 overall). 2) Administration of eNOS inhibitor, L-NAME, decreased vasoreactivity to E2 in CFs and CMs but not in TMs, indicating reduced eNOS-dependent vasodilation in TMs. 3) Acute testosterone exposure in CF arterioles (n=6) mimicked chronic T-GAHT effects of E2 in TMs. 4) Acetylcholine responses suggest normal endothelium-dependent vasodilation in TMs, with L-NAME significantly blunting the response (p<0.01 overall), implying impairment in other pathways. 5) NO donor, NONOate, responses were similar between TMs, CFs, and CMs, indicating normal smooth muscle function (n=3/group). 6) Flow-induced vasodilation was reduced in TMs (n=3) compared to CFs (n=4) and CMs (n=2) at higher pressures (p<0.05) while L-NAME significantly reduced responses in CFs, showed a decreasing trend in CMs, but had no effect in TMs.

These data indicate that resistance arterioles of TMs on T-GAHT exhibit impaired eNOS- and endothelial-dependent vasodilatory capacity, suggesting a potential vasodilator switch. Further research will elucidate specific contributions of NO, H2O2, and prostaglandins. These mechanistic insights are crucial for cardiovascular risk stratification in TMs on T-GAHT.