This study is being done to learn if yohimbine, a naturally occurring drug, affects the speed at which food travels through the stomach, intestines and colon, and if yohimbine affects the amount of liquid you can drink in a short period of time. Understanding how yohimbine works on the gut may help develop new treatments for patients with constipation. Yohimbine has been approved by the Food and Drug Administration (FDA) for routine clinical use, however, its use as proposed in this study is considered investigational.