Heart rate muscle tissue is largely influenced by hormones. The sympathetic system releases noradrenaline to speed up the heart rate, while the parasympathetic system releases acetylcholine to slow it down. But the release of these hormones is controlled by nerves, which are largely severed. So you typically end up with a less "dynamic" heart rate -- resting rate higher and responsiveness to stimuli reduced. These nerve connections can regenerate to some degree but it's individual and rarely close to what they were before.