What Young Rats’ Workouts Could Tell Us About the Human Heart
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Version 0 of 1. The human heart of song and story is a changeable, if not fickle, thing. But in one crucial sense, the organ has been seen as immutable. Scientists long thought that mammalian hearts stop producing most of their new cells shortly after birth, and that when they grow bigger, they do so primarily because the size of their existing cells increases. A recent study in The Journal of Physiology, however, confirms that exercise can substantially increase the number of cells in the hearts of young lab rats — and it also indicates, for the first time, that these additional cells are still present in mature hearts. Researchers in Australia took young, healthy male rats and kept some of them sedentary while making others run on treadmills. This active cohort was divided into three groups, each of which began exercising at a different stage of life. In human terms, these starting points corresponded to childhood, adolescence and adulthood. After the rats were put through a month of daily hourlong workouts at a moderate pace, the hearts of some of them were examined microscopically. Exercise was then curtailed for the rest of the rats, which spent the next several months (roughly equivalent to 10 human years) inactive. Once they reached full adulthood, their hearts were also scrutinized. The treadmillers had bigger hearts than their inactive counterparts, a finding expected by the researchers. Exercise makes hearts larger, more efficient and healthier. But the pathways to this change differed greatly depending on the rats’ ages when they started running. Those that began exercising as adults had bigger cardiac-muscle cells but not more of them. The hearts of the childhood runners, though, teemed with about 20 million additional cardiomyocytes — the type of heart cell that contracts — compared with the hearts of the sedentary rats of the same age. The adolescent runners also showed small increases in the number of cardiomyocytes, but many fewer than the youngest runners gained. Perhaps most interesting, the extra heart cells found in both young and adolescent rats remained in their hearts after they reached adulthood, despite the cessation of exercise. The study involved rodents, not people, so the findings shouldn’t be overhyped, cautions Glenn Wadley, an associate professor in the Institute for Physical Activity and Nutrition at Deakin University, who conducted the experiment with a graduate student, Yasmin Asif, and colleagues from the University of Melbourne and Monash University. But even so, the results do hint at human implications. A heart attack kills tens of millions of cells in an instant; a reserve of cardiac-muscle cells built up during childhood could conceivably affect the likelihood of surviving an attack as an adult, as well as influence how well a heart functions over the course of a lifetime. “Exercise is beneficial for the heart at any age,” Wadley says. His study provides “a new reason” for people to be active when their hearts are still young and unburdened. |