Do PromoCell HCMEC represent endocardial endothelial cells, i.e. the cells lining the cardiac ventricles? If so, why are they classified as microvascular endothelial cells?

No. PromoCell human cardiac microvascular endothelial cells (HCMEC) are not endocardial cells.

• Endocardial endothelial cells line the endocardium, the innermost layer of the heart that covers the cardiac chambers and ventricles.
• PromoCell HCMECs are isolated from the capillaries within the heart muscle (myocardium).

Because these cells originate from small blood vessels (capillaries) rather than from the endocardium, they are correctly referred to as microvascular endothelial cells.