Carry on Cleo. Two unusual glands: the mammary gland and the snake venom gland

I usually try to avoid writing about mammary glands and lactation since I have been doing that for nearly fifty years. However, I cannot resist drawing attention to a question I have been asking for nearly that  long.

Most exocrine glands, salivary glands for example, do not store their own secretion. They simply switch on when a supply of saliva, say, is needed. By contrast, the mammary gland secretes milk continuously and stores its secretion internally until milk is removed at intervals. Another gland—or glands—that share that characteristic is the snake venom gland which builds up a supply of venom to be ejected on striking prey or in defence. When Cleopatra killed herself by grasping a viper to her bosom, two unusual exocrine glands were brought into close apposition.

Embed from Getty Images

We know that the rate at which milk is secreted is controlled by the volume of milk removed. The more milk taken, the faster the rate of secretion. In other words, there is a feedback mechanism operating to match demand by the young to supply by the mother. But what about the snake venom gland? There is every reason to expect that the rate of venom secretion would be controlled. Venom, like milk, is expensive for the body to produce and one might predict that a snake with a full load of venom would not continue to produce more only for it to be broken down again. So the questions are: 1. Does the rate of venom secretion increase after a strike? 2. Does the rate decrease as the internal storage compartment fills? 3. If so, by what mechanism is such control of secretory rate exerted? There is evidence that the answer to the first of these two questions is ‘yes’.

This is Scott A. Weinstein, Tamara L. Smith, and Kenneth V Kardong writing in their review, Reptile Venom Glands. Form, Function, and Future, published in 2009 (references deleted):

Venomous snakes hold stored venom during extended periods of fasting, but it remains ready when feeding resumes after hibernation or in defense; there is no reported turnover of the stored venom protein. If manually depleted (extracted, or "milked"), the secretory epithelium of the main venom gland exhibits rapid protein synthesis with subsequent exocytosis replenishing venom stores in the ductules and large lumen. This process is completed in about 16 days. However, when expending venom during natural strikes, venom is replenished more rapidly, or less total venom is expended as judged by the rapid recovery of lethal envenomation of prey.

The question (3)—not an easy one to answer by experiment—remains of how the mechanism to control the rate of venom secretion works. Is feedback control exerted chemically, like the mammary gland in full lactation, or physically by increased pressure stretching the secretory epithelium, as can be induced in the mammary gland by the sudden cessation of milk removal?

From Weinstein et al. 2009

Weinstein SA, Smith TL, Kardong KV. 2009. Reptile Venom Glands. Form, Function, and Future. In, Handbook of Venoms and Toxins of Reptiles, edited by Stephen P. Mackessy, pages 65-91. Boca Raton: CRC Press.