I.1.9.4. Mechanism of action of antidiabetic medicinal plants
and their components
There are several possible mechanisms through which these
herbs can act to control the blood glucose level (Tanira,
1994). The mechanisms of action can be related, generally, to the
ability of the plant in question (or its active principle) to lower plasma
glucose level by interfering with one or more of the processes involved in
glucose homeostasis. The reported mechanisms whereby herbal antidiabetic
remedies reduce blood glucose levels are more or less similar to those of the
synthetic oral hypoglycemic drugs and are summarized as follows
(Tanira, 1994; Bastaki, 2005; Bnouham et
al ., 2006):
i) stimulation of insulin synthesis and/or secretion from
pancreatic beta-cells
ii) regeneration/revitalization of damaged pancreatic beta
cells
iii) improvement of insulin sensitivity (enhanced glucose
uptake by fat and muscle cells)
iv) mimicking the action of insulin (acting like insulin)
iv) alteration of the activity of some enzymes that are
involved in glucose metabolism
vi) slowing down the absorption of carbohydrates from the
gut.
I.1.9.5. Investigation of
some mechanism of action of antidiabetic plant extracts
Below is a brief description of some procedures used to
investigate the in vivo effects of plant materials on insulin
secretion, digestion and absorption of glucose, activation of the insulin
receptor and the activity of some carbohydrate metabolizing enzymes.
· Effect on
insulin secretion
In most published studies, investigation of the effect of
medicinal plant extract on insulin secretion in vivo has involved the
use of streptozotocin or alloxan induced animal models of diabetes
(Eidi et al., 2006). Both alloxan and streptozotocin
causes destruction of pancreatic beta cells resulting in reduced insulin
secretion (Fröde and Medeiros, 2008). In streptozotocin
and alloxan induced animal models of diabetes, insulin is markedly depleted but
not absent (Fröde and Medeiros, 2008). For this reasons
these animal models have been widely used to study the effect of antidiabetic
remedies on insulin secretion in vivo.
· Intestinal
digestion and absorption of carbohydrates
In order to investigate the effect of an antidiabetic plant
extract on intestinal digestion and/or absorption of carbohydrates, study
animals are usually divided into experimental and control groups. Experimental
animals are given a plant extract under investigation while control animals are
given a vehicle. An hour later, both groups of animals are given a fixed amount
of glucose, sucrose or starch. Thereafter, blood glucose levels are measured at
0.5, 1, 1.5, 2 and 3 hrs after administration of the carbohydrate. Areas below
the oral glucose tolerance curves of experimental groups are then calculated
and compared with those of control groups (Hannan et al.,
2007). Alternatively, a glucose tolerance test can be determined
in the same group of animals before and after oral administration of the plant
extract (Karato et al., 2006). A comparison of the
glucose tolerance curve before and after oral administration of the plant
extract will indicate whether or not the plant extract contribute to the delay
in carbohydrate digestion and subsequent lowering of the blood glucose level
glucose.
· Inhibition or
activation of carbohydrate metabolizing enzymes
It has been establish that some antidiabetic remedies, for
example, metformin exert its blood glucose effects by inhibiting endogenous
glucose production by the liver through the process of gluconeogenesis and
glycogenolysis (Bastaki, 2005). For this reason, as part of
efforts to find out the possible mode of action of antidiabetic remedies,
several researchers have investigated the effect of plant extracts on the
activities of gluconeogenic enzymes: glucose 6-phosphatase, fructose
1,6-bisphosphatase; the glycogenolytic enzyme; glycogen phosphorylase and
hepatic glucokinase. In order to investigate the effect of medicinal plant
extract on key enzymes involved in glucose homeostasis in vivo, the
study design used are similar to the one describe above for the study of the
effect of plant extract on stimulation of insulin except that at the end of the
feeding period blood and selected tissues are also collected for the
measurement of the activity of selected enzymes in plasma or tissue homogenates
in vitro.
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