Modulation of the Ca(2+) release channel of sarcoplasmic reticulum by amiloride analogs

Role of the Na+/Ca2+ exchanger in calcium homeostasis and human sperm motility regulation

A number of cell functions, such as flagellar beating, swimming velocity, acrosome reaction, etc., are triggered by a Ca2+ influx across the cell membrane. For appropriate physiological functions, the motile human sperm maintains the intracellular free calcium concentration ([Ca2+]i) at a submicromolar level. The objective of this study was to determine the role of the Na+/Ca2+ exchanger (NCX) in the maintenance of [Ca2+]i in human spermatozoa. Spermatozoa maintained in extracellular medium containing>or=1 microM Ca2+ exhibited motility similar to that of the control. In addition to several calcium transport mechanisms described earlier, we provide evidence that the NCX plays a crucial role in the maintenance of [Ca2+]i. Three chemically unrelated inhibitors of the NCX (bepridil, DCB (3',4'-dichlorobenzamil hydrochloride), and KB-R7943) all blocked human sperm motility in a dose and incubation time dependent manner. The IC50 values for bepridil, DCB, and KB-R7943 were 16.2, 9.8, and 5.3 microM, respectively. The treatment with the above-mentioned blockers resulted in an elevated [Ca2+]i and a decreased [Na+]i. The store-operated calcium channel (SOCC) inhibitor SKF 96365 also blocked the sperm motility (IC50=2.44 microM). The presence of the NCX antigen in the human spermatozoa was proven by flow cytometry, confocal laser scanning microscopy, and immunoblotting techniques. Calcium homeostasis of human spermatozoa is maintained by several transport proteins among which the SOCC and the NCX may play a major role.

Correolide, a nor-triterpenoid blocker of Shaker-type Kv1 channels elicits twitches in guinea-pig ileum by stimulating the enteric nervous system and enhancing neurotransmitter release

Correolide (1 - 10 microM), a nortriterpene purified from Spachea correae and a selective blocker of Kv1 potassium channels, elicits repetitive twitching in guinea-pig ileum. This effect is not seen in guinea-pig duodenum, portal vein, urinary bladder or uterine strips, nor in rat or mouse ileum. The time course and amplitude of the correolide-induced twitches in guinea-pig ileum are similar to those elicited by electrical stimulation of the enteric nervous system. The correolide-induced twitching is not affected by pre-treatment with capsaicin (1 microM), but is facilitated by the NO synthase inhibitor, N(G)-nitro-L-arginine methyl esther (L-NAME, 200 microM). The correolide-induced twitching is abolished by tetrodotoxin (1 microM) or hexamethonium (100 microM), and is markedly inhibited by nifedipine (0.3 microM) or atropine (0.2 microM). The atropine-resistant component is inhibited by selective antagonists of NK1 and NK2 tachykinin receptors, namely GR 82334 and GR 94800 (1 microM each). The former compound is more effective in inhibiting the correolide-induced, atropine-resistant activity. Correolide intensified the twitching of ileum segments exposed to saturating concentrations of margatoxin (MgTX), which suggests that Kv1 sub-types other than Kv1.1 (Kv1.4 or Kv1.5) are involved in the relatively greater degree of stimulation of the enteric nervous system by correolide, as compared to MgTX. We propose that blockade of Kv1 channels by correolide increases the excitability of intramural nerve plexuses promoting release of acetylcholine and tachykinins from excitatory motor neurons. This, in turn, leads to Ca(2+)-dependent action potentials and twitching of the muscle fibres.