From stimulators to inhibitors and destroyers of growth

Waller and Nowacki16 distinguished the role of alkaloids in plants as growth stimulators and inhibitors and also as protective agents and reservoirs of nitrogen. Some alkaloids are known neurotransmitters in animals and can also be considered part of the signalling system. This system is constructed as a part of cell and metabolic operations controlled by functional mechanisms of biological membranes, channels, receptors and enzymes. It is known that some alkaloids, for example purine and steroidal alkaloids, can bind to some compounds presented on cell membranes. As a result of this interactive process, the moiety segment of alkaloids can be changed by addition to different parts (e.g. lipophilic, hydrophilic etc.) of the molecule, which assists in binding to the receptor. There are different receptors for different compounds transported in the organism. Alkaloids can promote receptor activity or inhibit it. This is also in many cases connected with alkaloid moiety. The steroidal alkaloid gagamine, which has been isolated from the roots of Cynanchum wilfordi Hamsley (Asclepiadaceae), can be mentioned as an example. This alkaloid is known to have an inhibitory effect on the activity of aldehyde oxidase, which metabolizes heterocyclic rings336.

Alkaloids have their own signalling system. Receptors and membranes play an active role in this system. The role of biological membranes in alkaloid signalling is also connected with the action of the specific ion channels of

Ca2+, Na+ and K+ and their active pumps (e.g. Ca2+-ATPase). Only alkaloids can promote or inhibit activity of ion channels and their active pumps. Therefore, these channels are important in an alkaloid signalling system. This mechanism is connected directly or indirectly to receptor proteins. Alkaloids such as dopamine, histamine or serotonin are well-known neurotransmitters with their own receptors. The stimulation of a neurotransmitter system (especially ion channels) is caused by an influx of Na+-ions. This large-scale and rapid influx activates a so-called voltage gate of Na+ and K+-channels, which is essential for alkaloids. Neurotransmission is one of the most important biological characteristics of alkaloids. However, the latest research data published by Pineda et al.337 presents information about the effects of the crude extracts of lupine quinolizidine alkaloids, which were intracerebroventricularly administrated in adult rat brain tissue. These extracts were administrated to the right lateral ventricle of adult rats through a stainless steel cannula for five consecutive days. The researchers stated in their report that immediately after the administration of quinolizidine alkaloid from Lupinus exaltatus and Lupinus montanus seeds, the rats began grooming and suffered from tachycardia, tachypnea, piloerection, tail erection, muscular contractions, loss of equilibrium, excitation and an unsteady gait. Moreover, Pineda et al.337 reported that the rats treated with alkaloids had damaged neurons. Although there was no statistical significance, damages were observed and may suggest a histo-pathological influence on neurons. The most frequent abnormalities observed in this brain tissue were the "red neurons" with a shrunken eosinophilic cytoplasm, strongly stained pyknotic nuclei, neuronal swelling, spongiform neuropil, "ghost cells" (hypochromasia) and abundant neu-ronophagic figures in numerous brain areas. If these results will be proved in the future by no direct administration of alkaloids to the brain, they will serve as evidence of the destructive role of alkaloids in the animal body. Although the research of Pineda et al.337 is interesting in many aspects, the results cannot be considered as evidence of such destruction caused by alkaloids. It is evidently known that crude extracts cannot be physiologically transported to the animal brain. The direct administration of crude extracts to the brain tissues from outside the liver affects the influence of all components of these extracts, from which alkaloids are only one part.

There is evidence in literature that alkaloid biology is connected with regulation, stimulation and induction functions. Tsai et al.338 proved that caffeine levels in the blood, brain and bile of rats decreased when given a treatment of rutaecarpine, an alkaloid from Evodia rutaecarpa (Figure 78). It is known that caffeine has been found to enter the brain by both simple diffusion and saturable carrier-mediated transport339. The hepatobiliary excretion of caffeine has also been reported in humans341, rabbits340 and rats338.

A treatment of rutaecarpine causes an increase in renal microsomal enzymes related to CYP1A and enhances the activity and protein levels of CYP1A. It is known that caffeine is a mild stimulant. It is metabolized in the liver by

Figure 78. Rutaecarpine, an alkaloid from Evodia rutaecarpa.

CYP1A2, and it also has been shown to be an inducer of CYP1A2 in rodents on account of the increase in hepatic microsomal CYP1A2342. Rutaecarpine is an inducer of cytochrome P450(CYP)1A in mouse liver and kidney343.

There is evidence that alkaloids influence plant growth, as both stimulators and regulators. A large series of applied studies in Germany and in Poland started in the 1980s proved that quinolizidine alkaloids in crude lupine extracts had effects on both yield amount and quality (Figure 79). Foliar application of lupine extract on several crops resulted in yield increases of 17-20%331332 and 15-25%235 333. Moreover, these results proved that crude lupine extract with quinolizidine alkaloids influenced the balance of nitrogen compounds in plants. Increases in protein concentration and changes in amino acid contents have been observed. Snap bean (Phaseolus vulgaris L.) seed yield after foliar application of the extract increased by 16.4% and the biological value of protein measured with essential amino acid coefficients increased by 2.87%333. The stimulation role of alkaloids can be explained by more intense nitrogen metabolism after application. In the 1950s a case of applying pure lupanine solution to the leaves of alkaloid-poor Lupinus albus L. was shown to have a growth-stimulating effect16. However, there are also old findings indicating some plants exhibited no effects at all when treated. In some cases they exhibited growth inhibition or the effects of poisoning.

Evidence of the alkaloidal plant growth stimulation Effects of foliar application of lupine extracts on

- cereals

- sugar beets

Yield increase 17-20%

- maize

- potato

Change of

- total aminoacids

- essential aminoacids

- snap bean

Figure 79. Effects of foliar application of lupine extracts. Sources: Refs [331, 332, 333, 334, 335].

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