Saliva contains many enzymes and proteins that serve a variety of biological functions, including initiating digestion and controlling the mouth’s environment. The latter may be important in the development of tooth decay. The protein content of saliva seems to be regulated by sympathetic nerves, a component of the autonomic nervous system. β1-

Receptors for catecholamines released by sympathetic nerves have been shown to be important, but little is known about the role of β2-receptors.

To better define the relative importance of these two receptor types in regulating salivary proteins, particularly those proteins that contain large amounts of the amino acid proline, Johnson and Cortez* gave five groups of rats one of four drugs or a saline injection (control). The four drugs were isoproterenol, a nonselective β-stimulant that affects both types of receptors; dobutamine, a stimulant selective for β1-receptors;

terbutaline, a stimulant selective for β2-receptors; and metoprolol, a blocker selective for β1-receptors. They then measured the proline-rich protein content of the saliva produced by one of the salivary glands (the data are in Table D-20, Appendix D). Which receptor type, if any, seems to control the content of proline-rich proteins in rat saliva?