The Endothelial Barrier Restricts Endocrine Actions to the Luminal Vascular Receptors: Changing the Paradigm: A Didactic Approach

In 1849, the first list of endocrine hormones was discovered and proposed that the synthesizing gland delivers it to the circulation.  The circulatory hormone reaches the target organ, physically unimpeded acts directly on the parenchymal cells. Such a simplistic view persists despite new knowledge of an endothelial wall barrier and implications for every parenchymal cell in the body. This misconception leads to inadequate interpretations of data, wrong diagnosis and therapeutic expectations, erroneous hypotheses, and misleads further research work. The quest of this review is to play down this misconception by pointing out key overlooked findings of the vascular endothelial wall: 1) The selective endothelial barrier physically separates two same-hormone-containing compartments; the endocrine and the interstitial autocrine hormone compartments, 2) the hormone concentrations values in these compartments are independent of each other, 3) in each compartment the hormone acts solely on the receptors of that particular compartment, 4) multiple intravascular endocrine hormones act solely on their corresponding luminal endothelial membrane receptor (LEMR), without directly acting on the parenchymal cells, 5) Agonist-activation of LEMR triggers the release of specific paracrine endothelial agents that in conjunction with autocrine interstitial hormone modulate parenchymal function(s) and perhaps the turnover of the interstitial autocrine hormone, 6) these hormone compartments, functionally interact via paracrine exchange signaling, and the integrated intercourse of all these events result in the final hormonal organ effect. The present challenges to achieving more rationale therapeutic effects are to design agonists or antagonists that exclusively gain access to a target compartment and have high specificity for the receptor of the cells in that compartment.