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But the hormones secreted by the larger front lobe make the pituitary the "master gland." Yet no one knows just what or how many hormones the front lobe secretes. The trouble is that—although a few front lobe hormones have been freed of all but small traces of impurities—a living body's response under test may be due not to the hormone tested but to an "impurity."
Moreover, some responses are not due to one hormone, but to the combined action of two or more. This phenomenon is called "synergism." The work of Evans and others convinced many a gland man that the pituitary secreted a "growth hormone." Riddle's researches, however, tend to show that growth is a synergic response to two hormones, thyrotropin and prolactin. He also believes that this same pair get together in another synergism to maintain body heat.
The pituitary spurs other glands to produce hormones of their own, and in some cases these secretions may return to act on the pituitary. Thus primary, secondary and tertiary responses can be traced to the gland.
When these sources of confusion were less well understood than now, it was fashionable to attribute every pituitary reaction to a separate hormone. Riddle exposed some of these as "ghosts." He believes the number of definitely established front-lobe hormones to be small—five at most. These possible five are 1) thyrotropin, the thyroid-stimulator; 2) adrenotropin, which acts on the adrenals; 3) and 4), "FSH" and "LH" which affect the ovaries or testes; 5) prolactin.
Mother Love. Six years ago, by a repeated complicated procedure called "isoelectric precipitation," the Riddle laboratory crew obtained a pituitary substance which did not noticeably affect the thyroid or the sex glands, but which had a marked effect on the mammary glands. It started milk production not only in normal female guinea pigs but also in spayed females and males. In pigeons it thickened the crop sac, which provides a liquid ("pigeon milk") with which pigeons feed their young. Riddle called this new hormone prolactin.
Prolactin caused chickless hens and virgin rats to mother chicks and ratlets with great affection. This earned prolactin the nickname of "mother love" hormone. The implication of the discovery was that mother love, though doubtless fortified and colored in women by training and tradition has a physiological basis in a chemical substance—probably large protein molecules. (Dr. Riddle found that prolactin and other front-lobe hormones are disintegrated by trypsin, an enzyme which has the special property of "digesting" proteins.)
Busy Hormone. Prolactin also inhibits the activity of the sex glands, which is obviously nature's way of quieting the distraction of sex urges when parental behavior and responsibility are called for. And either alone or in concert with other hormones, it maintains the weight of abdominal organs; controls basal metabolism (heat production) ; promotes appetite; and probably also affects the metabolism of carbohydrates and fat.