The Concept of Cellular Tone: Reflections on the Endothelium, Fibroblasts, and Smooth Muscle Cells

THE CONCEPT OF CELLULAR TONE: REFLECTIONS ON THE ENDOTHELIUM, FIBROBLASTS, AND SMOOTH MUSCLE CELLS CARL A. BOSWELL, ISABELLE JORIS, and GUIDO MAJNO* What Is Tone? Tone is currently understood as a property of muscular tissue, smooth as well as striated. The purpose of this essay is to explore the meaning of tone at the level of cells, both muscular and non-muscular. The two basic meanings of the word tone—musical as well as mechanical —have not changed since the tónos of the ancient Greeks, which stood for pitch as well as for tension [I]. In Guyton's authoritative Textbook ofMedical Physiology, tone is discussed in relation to smooth muscle [2]: Smooth muscle can maintain a state of long-term, steady contraction that has been called either tonus contraction of smooth muscle or simply smooth muscle tone. This is an important feature of smooth muscle contraction because it allows prolonged or even indefinite continuance of the smooth muscle function. For instance, the arterioles are maintained in a state of tonic contraction almost throughout the entire life of the person. Guyton then proceeds to explain that tonic contractions of smooth muscle can be caused in one of two ways: (1) sometimes by summation of individual contractile pulses, each one initiated by a separate action potential, or (2) more often by prolonged direct smooth muscle excitation without action potentials, caused by local or by circulating hormones . All this means that the concept of tone as a prolonged or even indefinite state of contraction is well established for smooth muscle as a tissue. But then, if the tissue of an arteriole can be in a state of semi-permanent This work was supported by NIH grant HL-25973. The authors thank Ms. Anne H. Cutler for technical assistance and Ms. Jane M. Manzi for manuscript preparation. ""Department of Pathology, University of Massachusetts Medical Center, 55 Lake Avenue North, Worcester, Massachusetts 01655.© 1992 by The University of Chicago. All rights reserved. 003 1-5982/93/360 1-0789$0 1 .00 Perspectives in Biology and Medicine, 36, 1 ¦ Autumn 1992 | 79 contraction, where does that contraction come from? We will argue that it can only come from the individual smooth muscle cells. In other words, tone must exist also at the level of cells. This concept appears self-evident, but we have not seen it expressed; it seems to have been lost in the gap between whole-organ physiologists and single-cell biologists . Since endothelial cells and fibroblasts are also known to be contractile , we will further argue that they should also be able to maintain a long-lasting tone. Tone at the Level of Cells We came across cellular tone in a series of experiments on cultured cells. Our original purpose was to challenge three types of contractile cells (endothelium, smooth muscle cells, and fibroblasts) with contracting and relaxing agents, and to compare their responses [3]. To do so we adopted a clever system devised a decade ago [4]. The principle is to seed cells on a membrane thin enough to be wrinkled if the cell contracts. The membrane itself is prepared by spreading a drop of silicone on a glass surface, then quickly passing it through a flame, so that the surface polymerizes whereas the silicone beneath it remains fluid. The cells, in our experience, have some difficulty in attaching to the silicone membrane, but they can be coaxed to do so if the surface is treated appropriately, i.e., with Cell-Tak (Collaborative Research, Inc., Bedford, MA) and gelatin, plus laminin and fibronectin for endothelial cells. We used aortic endothelium and smooth muscle cells from bovines and from rats, as well as fibroblasts from rat renal capsule and from calf dermis. Accordingly, we seeded the three types of cells on silicone membranes and waited for them to settle, that is, to become firmly attached. As soon as that happened, the plan was to stimulate the cells with suitable contracting agents, such as bradykinin or serotonin. However, an unforeseen event frustrated our plans: the cells did settle, but in doing so they also contracted. Beautiful wrinkles appeared in the silicone substrate (Figure 1). Similar findings were reported by Kelley, et al...