The Effects of Mechanical Stimulation on Vertebrate Hearts

Abstract

All vertebrate cardiac muscle responds intrinsically to mechanical stimulation which can lead to changes in both the inotropic and chronotropic state of the heart. However the magnitude and physiological relevance of these mechanically-induced responses differ between vertebrate classes. This review will discuss the differences and similarities in the response of vertebrate hearts to stretch. It will focus on responses to mechanical stimulation that have been well characterised in mammals, and discuss what is known about them in non-mammalian vertebrates. Specifically we focus on the Frank- Starling response or length-tension relationship, stretch acceleration of heart rate (the Bainbridge effect) and mechanically-induced effects on cardiac rhythm. Although they have not been categorically studied, these three basic mechanical and electrical responses to stretch are likely present in all vertebrate classes. For example, in a manner similar to mammals, one of the earliest vertebrates, the hagfish (Myxine glutinosa), shows a remarkable increase (up to 150%) in heart rate in response to cardiac stretch and in amphibian hearts, modification of action potential profiles and mechanically triggered action potentials have been observed. These commonalities are interesting given the differences in whole heart, cellular and sub-cellular morphology and working environments between early and later vertebrates. These differences may have led to fundamental differences in cardiovascular design between classes. For instance, the exquisite sensitivity of the Starling response in the heart of the rainbow trout (Oncorhynchus mykiss) may explain why fish increase predominantly stroke volume rather than heart rate when modulating cardiac output. We speculate that despite the variety of vertebrate hearts, mechanosensitivity is fundamentally similar, if subtly different, across classes. Shared mechanisms, such as mechanosensitive channels (possibly TRP), make early vertebrate hearts useful models for the study of cardiac mechanosensitivity. Finally, given that early vertebrates are known to rely on intrinsic regulation to a greater extent than later vertebrates they may provide useful systems in which to study the role of mechanosensitivity in the evolution of cardiac function and cardiac regulation.