Chemical extirpation of putative endocrine tissues can be performed by the use of substances that inhibit the formation and release of the endocrine secretions, or block the effector sites. The formation of thyroid hormones can be prevented by administering radioactive 131 1which can destroy the tissue, or by giving drugs, like thiourea and propylthiouracil, that prevent hormone synthesis. Adrenocortical steroid formation can be inhibited by
metyrapone, while insulin and glucagon production are blocked respectively by, alloxan
and synthalin A or cobalt chloride. Such substances, however, may not be very specific,
and can have widespread toxic effects.
The release of many hormones into the circulation can also be prevented ; ethyl alcohol
inhibits the release of neurohypophysial antidiuretic hormone, while gonadal and adrenocortical steroids reduce the release of their appropriate adenohypophysial trophic hormones
by a negative feed-back action. Subsequent to its release, the action of a hormone may be
prevented by antagonisms at receptor-effector sites; spirolactones, like aldactone, competitively inhibit the action of aldosterone on the kidney and anuran urinary bladder, while
structural analogues of the neurohypophysial hormones may also block the action of the
parent hormone. The possibilities of immunologically inhibiting endocrine function with
specific antibodies are only just beginning to be investigated.
c) Administration of tissue extracts
Tissue extracts often have well defined biological effects in vivo and in vitro, and such observations may play an important role in assessing the putative endocrine role of tissues .
Such decoctions may be administered as an attempted replacement procedure following extirpation of the alleged gland, their actions may be observed in intact animals or they may
be tested on isolated preparations like the uterus and anuran skin and urinary bladder.
The initial problem, after obtaining sufficient of the desired tissue, is how to extract
it; simple saline homogenates are usually tried out in the first instance. These decoctions
may be subsequently modified in attempts to separate any active material from the multitude
of other substances that are usually present. If no activity is found in this way, alternate
extraction procedures may be tried, as the substance may be poorly soluble in aqueous solution, or even fail to survive such an apparently simple treatment. Thus extraction in such
solvents as alcohols and acetone may be attempted.
There are many difficulties in assessing the significance of biological evidence obtained from the effects of such extracts. Lack of a demonstrable effect is inconclusive, as
the extraction procedure may be inadequate, the amount of material stored in the tissue may
be insufficient to have a stimulatory action, or the biological parameters being examined
may be inappropriate. Early attempts to decoct insulin from pancreas were unsuccessful, due
to its inactivation by the trypsin which is present in the associated exocrine tissue . It was
subsequently found that the enzyme's action could be prevented if ethyl alcohol was present,
a procedure which leaves the insulin intact . The storage of corticosteroids is poor, so that
large numbers of glands originally had to be used in order to show their biological actions .
A suitable choice -of the route for administration can assist in instances when only small
amounts of material are available; direct injection into a blood vessel supplying the sUPFosed
'target' organ, or the creation of regional storage depots may provide adequate loca concentrations. The condition of an animal may be unsuitable for the demonstration of an hormonal effect; thus the neurohypophysial antidiuretic hormone was originally shown to have
a diuretic effect when tested on anaesthetized mammals and it was only when hydrated unanaesthetized animals were used that the antidiuretic action became apparent. This problem
arose as a result of the high endogenous circulating levels of ADH that are present in animals
exposed to many anaesthetics. Apart from physically removing the source of glandular secretion, high endogenous levels of hormones can often be reduced by providing conditions
calculated to reduce their release; for instance administration of water reduces circulating
ADH levels, and placing toads in hypotonic sodium chloride solutions for several days lowers their plasma aldosterone levels.
The biological action of a tissue extract can reflect its physiological role, but such evidence alone is not conclusive, and at the most is only suggestive. Pharmacological effects
of such extracts are more common than any physiological actions . Mammalian antidiuretic
42
metyrapone, while insulin and glucagon production are blocked respectively by, alloxan
and synthalin A or cobalt chloride. Such substances, however, may not be very specific,
and can have widespread toxic effects.
The release of many hormones into the circulation can also be prevented ; ethyl alcohol
inhibits the release of neurohypophysial antidiuretic hormone, while gonadal and adrenocortical steroids reduce the release of their appropriate adenohypophysial trophic hormones
by a negative feed-back action. Subsequent to its release, the action of a hormone may be
prevented by antagonisms at receptor-effector sites; spirolactones, like aldactone, competitively inhibit the action of aldosterone on the kidney and anuran urinary bladder, while
structural analogues of the neurohypophysial hormones may also block the action of the
parent hormone. The possibilities of immunologically inhibiting endocrine function with
specific antibodies are only just beginning to be investigated.
c) Administration of tissue extracts
Tissue extracts often have well defined biological effects in vivo and in vitro, and such observations may play an important role in assessing the putative endocrine role of tissues .
Such decoctions may be administered as an attempted replacement procedure following extirpation of the alleged gland, their actions may be observed in intact animals or they may
be tested on isolated preparations like the uterus and anuran skin and urinary bladder.
The initial problem, after obtaining sufficient of the desired tissue, is how to extract
it; simple saline homogenates are usually tried out in the first instance. These decoctions
may be subsequently modified in attempts to separate any active material from the multitude
of other substances that are usually present. If no activity is found in this way, alternate
extraction procedures may be tried, as the substance may be poorly soluble in aqueous solution, or even fail to survive such an apparently simple treatment. Thus extraction in such
solvents as alcohols and acetone may be attempted.
There are many difficulties in assessing the significance of biological evidence obtained from the effects of such extracts. Lack of a demonstrable effect is inconclusive, as
the extraction procedure may be inadequate, the amount of material stored in the tissue may
be insufficient to have a stimulatory action, or the biological parameters being examined
may be inappropriate. Early attempts to decoct insulin from pancreas were unsuccessful, due
to its inactivation by the trypsin which is present in the associated exocrine tissue . It was
subsequently found that the enzyme's action could be prevented if ethyl alcohol was present,
a procedure which leaves the insulin intact . The storage of corticosteroids is poor, so that
large numbers of glands originally had to be used in order to show their biological actions .
A suitable choice -of the route for administration can assist in instances when only small
amounts of material are available; direct injection into a blood vessel supplying the sUPFosed
'target' organ, or the creation of regional storage depots may provide adequate loca concentrations. The condition of an animal may be unsuitable for the demonstration of an hormonal effect; thus the neurohypophysial antidiuretic hormone was originally shown to have
a diuretic effect when tested on anaesthetized mammals and it was only when hydrated unanaesthetized animals were used that the antidiuretic action became apparent. This problem
arose as a result of the high endogenous circulating levels of ADH that are present in animals
exposed to many anaesthetics. Apart from physically removing the source of glandular secretion, high endogenous levels of hormones can often be reduced by providing conditions
calculated to reduce their release; for instance administration of water reduces circulating
ADH levels, and placing toads in hypotonic sodium chloride solutions for several days lowers their plasma aldosterone levels.
The biological action of a tissue extract can reflect its physiological role, but such evidence alone is not conclusive, and at the most is only suggestive. Pharmacological effects
of such extracts are more common than any physiological actions . Mammalian antidiuretic
42