Interaction of tetrandrine with slowly inactivating calcium channels. Characterization of calcium channel modulation by an alkaloid of Chinese medicinal herb origin

New ligands for L-type Ca2+ channels

Although the clinically important categories of drug represented by verapamil, nifedipine and diltiazem are defined as acting at three major and discrete sites on the L class of voltage-dependent Ca2+ channel, it is likely that some new classes of drug modulate channel activity by acting at additional sites. David Rampe and David Triggle describe the actions of some of these new drugs, which may both offer improved therapeutic and side-effect profiles over existing agents and provide information to define further the structure and function of this channel class. These drugs may mimic the actions of endogenous ligand(s) and such ligands could provide new directions for Ca2+ channel drug structures.

Structure and hypotensive activity relationships of tetrandrine derivatives in stroke-prone spontaneously hypertensive rats

1. Structure and hypotensive activity relationships of tetrandrine (TD), an alkaloid isolated from the Chinese herb Radix stephaniae tetrandrae and its derivatives were investigated in conscious stroke-prone spontaneously hypertensive rats (SHRSP). 2. Derivatives substituted at the 7-O position with various types of alkyl group produced varying degrees of hypotensive effect. 3. While the demethylated derivative, fangchinoline (FC), and its acetylated compound had no effect on blood pressure, 7-O-methyl FC (TD), and 7-O-ethyl and 7-O-isopropyl FC at oral doses of 25 and 50 mg/kg produced a gradual and sustained hypotensive effect without any significant effects on heart rate and plasma renin concentration. 4. Substitution at the 7-O position with longer side chains such as n-propyl, n-butyl and n-pentyl groups reduced both the degree and duration of hypotensive activity. 5. Substitution of N-methyl groups at the 2 and 2' positions with quaternary ammonium or N-oxide attenuated the hypotensive activity. 6. The results of this study suggest a possibility that 7-O-ethyl and 7-O-isopropyl derivatives as well as TD can be considered as potential antihypertensive drugs because of the gradual onset and long duration of their hypotensive action in SHRSP.

Inhibition of prostaglandin and leukotriene generation by the plant alkaloids tetrandrine and berbamine

We compared the effects of two bisbenzylisoquinoline compounds on leukotriene and prostaglandin generation by human monocytes and neutrophils. The results show that tetrandrine had a much greater effect than berbamine on leukotriene generation. However, both compounds were equally potent in suppression of prostaglandin generation. This inhibitory effect on prostaglandin generation can be overcome by exogenous arachidonic acid (AA), suggesting that the site of inhibition is not on the cyclooxygenase enzyme complex, but more proximally on the phospholipase-mediated release of AA from the cell membrane, similar to the action of corticosteroids. These results, together with previous findings of inhibitory effects on other inflammatory mediators such as histamine, platelet-activating-factor (PAF) and interleukin 1 (IL-1) indicate that these plant alkaloids may be useful lead compounds for the development of a new class of anti-inflammatory drugs.

Inhibition of the liver cell receptor-activated Ca2+ inflow system by metal ion inhibitors of voltage-operated Ca2+ channels but not by other inhibitors of Ca2+ inflow

The properties of the receptor-activated Ca2+ inflow system in the liver cell plasma membrane were compared with those of voltage-operated Ca2+ channels and receptor-operated Ca2+ channels present in other cell types by testing the susceptibility of the Ca2+ inflow system to inhibition by other metal ions and known inhibitors of Ca2+ movement across membranes. Co2+ inhibited Ca2+ inflow through the receptor-activated Ca2+ inflow system, as assessed by measurement of (a) the activation by extracellular Ca2+ (Cao2+) of glycogen phosphorylase in the presence of vasopressin and (b) 45Ca2+ exchange in the presence of the hormone. The concentration of Co2+ which gave half-maximal inhibition was 280 microM. The inhibition by Co2+ was reversed by high Cao2+. Co2+ did not inhibit basal Ca2+ inflow as measured by 45Ca2+ exchange in the absence of vasopressin. Zn2+, Cd2+, Ni2+ and Mn2+ each inhibited Ca2+ inflow through the receptor-activated Ca2+ inflow system. The concentrations of these ions which gave half-maximal inhibition were 10, 50, 220 and 400 microM, respectively. Little inhibition of receptor-activated Ca2+ inflow was observed in the presence of Sr2+ or Ba2+. However, substantial amounts of 90Sr2+ were taken up by hepatocytes. Rates of 90Sr2+ uptake increased from 0.5-8 nmol per min per mg wet wt. when the extracellular concentration of Sr2+ was varied from 0.25 to 2.5 mM. Sr2+ uptake was inhibited 50% by Cao2+ with half-maximal inhibition at 100 microM Cao2+, but was not inhibited by verapamil and was not stimulated by vasopressin. The movement of Ca2+ through the receptor-activated Ca2+ inflow system was not inhibited by high concentrations of each of a number of inhibitors of voltage-operated and receptor-operated Ca2+ channels and intracellular Ca2+ movement. It is concluded that while the susceptibility to inhibition by metal ions of the receptor-activated Ca2+ inflow system in the liver cell plasma membrane is similar to that of voltage-operated Ca2+ channels, there are significant differences between the liver cell receptor-activated Ca2+ inflow system and both voltage-operated Ca2+ channels and some other receptor-operated Ca2+ channels with respect to inhibition by organic compounds.

Selective antimalarial activity of tetrandrine against chloroquine resistant Plasmodium falciparum

Antimalarial activity of tetrandrine was studied using a continuous in vitro culture of Plasmodium falciparum. Experimental results showed that tetrandrine has potent antimalarial effect on both chloroquine sensitive and resistant strains of Plasmodium falciparum. Interestingly, tetrandrine is about three times more potent against the chloroquine resistant strain than it is against the sensitive strain based on their IC50 values, which were 5.09 x 10(-7) M for the sensitive strain and 1.51 x 10(-7) M for the resistant strain. In addition, reversal experiments revealed that tetrandrine cannot reverse chloroquine-resistance, although it has verapamil-like, calcium-channel-blocker activity.