. "The regulation of the rate of heart contraction mediated by chemical signaling, hormonal, autocrine or paracrine." . . "regulation of heart rate by chemical signal" . "chemical cardiac chronotropy" . "chemical signal regulation of heart contraction rate" . "chemical signal regulation of heart rate" . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .