No guidelines for vascular nerves?

Where have all vascular nerves gone?

  Long time passing.

Where have all vascular nerves gone?

  Long time ago.

Where have all vascular nerves gone?

  Vascular biologists ignore them everyone.

Oh, When will they ever learn?

  When will they ever learn?

After Pete Seeger (1955)

to the editor: In a recent issue of the American Journal of Physiology-Heart and Circulatory Physiology, Camilla Wenceslau and 14 coauthors from various North American laboratories report guidelines for the measurement of vascular function and structure in isolated arteries and veins (1). The article describes in great detail procedures to isolate blood vessels, record their vasomotor responses, and quantify myogenic tone, endothelium-dependent vasodilator responses, and influences of perivascular adipose tissue. It does not address investigation of vascular nerves that are important for cardiovascular function. It is restricted to local aspects of vascular physiology.

The review by Wenceslau et al. (1) is part of a growing number of responses to the policy of the American Physiological Society to improve scientific rigor and transparency in physiological research (2). The authors aim to provide a comprehensive document detailing best practices and pitfalls for measurement of function and structure in large (conduit) and small (resistance) arteries and veins. According to the authors, scientists will have consistency among methodological approaches following their recommendations. Consequently, their results will be reproduced and validated by other vascular physiologists. Also, by following these guidelines, researchers will get rigorous in their scientific approach, collect reliable data, and enhance peer and public perception of science. These goals are noble. To meet these goals, ideal guidelines should be complete, correct, based on a broad consensus, and open to growing new insights.

Detailed technical practicalities are important in ex vivo vascular and other cardiovascular research activities. It can, however, be debated whether guidelines should be restricted to collections of standard operating procedures and experimental protocols for selected machines that presently dominate the field. In our opinion, it is also important to share with future vascular biologists the physiological and pharmacological reasonings that have contributed to these choices. The aim of the guidelines can then extend from 1) how should I use this piece of equipment to 2) how can I contribute to a better understanding of vascular function in health and disease. The review by Wenceslau et al. (1) starts with reference to De Motu Cordis, by William Harvey (1628), to introduce the reader to the anatomy and physiology of the vasculature. Also important for the review of the structure and function of isolated arteries and veins are studies by Sir Henry Dale (https://www.nobelprize.org/prizes/medicine/1936/dale/lecture/) and many others, during the first decades of the previous century. They investigated influences of circulating hormones and of the peripheral nervous system on cardiovascular function in intact experimental animals. Ex vivo analyses of motor responses of isolated blood vessels and other isolated tissues emerged from these studies to enhance throughput [e.g., Ahlquist (3)] and in an attempt to discriminate between nervous, hormonal, and local influences on vascular function.

In the intact human and experimental animal, the peripheral nervous system is tonically active and has considerable effects on total peripheral vascular resistance, the distribution of cardiac output, cutaneous vasomotor tone and thus thermoregulation, and a wide variety of other vascular functions. In vivo, the activity of peripheral nerves to the vasculature (i.e., the frequency of action potentials traveling along vascular nerves) varies considerably as it is modulated at several levels (afferent nerves, central nervous system, peripheral ganglia, and nerve endings/varicosities) by a broad variety of mechanical factors and chemical mediators (4). When blood vessels are removed from the body, they are disconnected from the peripheral ganglia and are thus removed from these variable tonic nervous influences. Yet, isolated blood vessels are not denervated. Nerve fibers run within the wall of blood vessels with densities that differ along the vascular trees and vary between vascular beds (5, 6). These vascular nerve fibers remain present and functional for several hours after isolation of blood vessels from the body.

Until 50 years ago, postganglionic sympathetic nerve fibers that release noradrenaline within the wall of blood vessels were considered the only vascular neuroeffector mechanism operating in arteries and veins (4, 7). To this were progressively added the sympathetic cotransmitter effects of ATP and neuropeptide Y (6), vascular sensory-motor nerves releasing calcitonin gene-related peptide (CGRP), and substance P mediating nonadrenergic-noncholinergic (NANC) vasodilatation (8), and NO-releasing vasodilator nerves (9). Contributions of these vascular nerves and their interactions to local and systemic vascular functions in health and disease are still only partly understood (5, 10).

Not considering vascular nerves in guidelines for in vitro (1) and in vivo (11) cardiovascular research may have several consequences. The interest in local phenomena (e.g., myogenic tone and endothelium-dependent vasodilatation) increases at the expense of systemic integrative physiology. This concern is growing throughout the physiological research community (12). At the other extreme, the scientific rigor and reproducibility of the experimental approaches that are detailed by the authors with the aim to be copy-pasted by future researchers, are compromised. Many tools that are used to study functions of vascular smooth muscle and endothelium affect vascular nerves as well. To name a few, these include buffer with elevated concentration of potassium ions (at the expense of sodium), ouabain, and other inhibitors of electrogenic sodium transport, and analogs of l-arginine that inhibit NO synthases.

A future list of guidelines to study vascular nerves in vitro and in vivo can only partly alleviate the concerns mentioned in this Letter. Ideal guidelines should be complete, correct, and open to growing new insights based on a broad consensus. The necessary flexibility, debate, volume, and detail of the information required to accomplish this, are hard to accommodate within the frame of a regular scientific paper or review like that by Wenceslau et al. (1) and others responding to the scientific rigor policy of AJP (2, 12). Ultimately, a web-based tool such as the IUPHAR/BPS Guide to Pharmacology (www.guidetopharmacology.org) may be better suited for present and future purposes.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.G.R.D.M. drafted manuscript; J.G.R.D.M., U.S., and C.A. edited and revised manuscript; J.G.R.D.M., U.S., and C.A. approved final version of manuscript.