Receptor-mediated increases in intracellular Ca2+ levels can be caused by release from intracellular organelles and/or influx from the extracellular fluid. Noradrenaline (NA) released from sympathetic nerves acts on alpha 1-adrenoceptors to increase cytosolic Ca2+ and promote smooth muscle contraction. In many cells activation of alpha 1-adrenoceptors causes formation of inositol 1,4,5-trisphosphate which promotes Ca2+ release from intracellular stores. The mechanism by which receptor activation opens cell surface Ca2+ channels is not known, although in some cases it may be secondary to formation of inositol phosphates or release of stored intracellular Ca2+ (ref. 3). However, alpha 1-adrenoceptors have recently been shown to have different pharmacological properties in different tissues, and it has been proposed that different alpha 1-adrenoceptor subtypes may control mobilization of intracellular Ca2+ and gating of extracellular Ca2+ influx. We here report evidence for two subtypes of alpha 1-adrenoceptors which cause contractile responses through different molecular mechanisms. One subtype stimulates inositol phosphate (InsP) formation and causes contractions which are independent of extracellular Ca2+, and the other does not stimulate inositol phosphate formation and causes contractions which require the influx of extracellular Ca2+ through dihydropyridine-sensitive channels. These results suggest that neurotransmitters and hormones may control Ca2+ release from intracellular stores and influx through voltage-gated membrane channels through distinct receptor subtypes.

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Marine sponges are currently one of the richest sources of pharmacologically active compounds found in the marine environment. These bioactive molecules are often secondary metabolites, whose main function is to enable and/or modulate cellular communication and defense. They are usually produced by functional enzyme clusters in sponges and/or their associated symbiotic microorganisms. Natural product lead compounds from sponges have often been found to be promising pharmaceutical agents. Several of them have successfully been approved as antiviral agents for clinical use or have been advanced to the late stages of clinical trials. Most of these drugs are used for the treatment of human immunodeficiency virus (HIV) and herpes simplex virus (HSV). The most important antiviral lead of marine origin reported thus far is nucleoside Ara-A (vidarabine) isolated from sponge Tethya crypta. It inhibits viral DNA polymerase and DNA synthesis of herpes, vaccinica and varicella zoster viruses. However due to the discovery of new types of viruses and emergence of drug resistant strains, it is necessary to develop new antiviral lead compounds continuously. Several sponge derived antiviral lead compounds which are hopedto be developed as future drugs are discussed in this review. Supply problems are usually the major bottleneck to the development of these compounds as drugs during clinical trials. However advances in the field of metagenomics and high throughput microbial cultivation has raised the possibility that these techniques could lead to the cost-effective large scale production of such compounds. Perspectives on biotechnological methods with respect to marine drug development are also discussed.

␣ 1 -Adrenergic receptors (ARs) belong to the large Class I G protein-coupled receptor superfamily and comprise three subtypes (␣ 1A , ␣ 1B , and ␣ 1D ). Previous work with heterologously expressed C-terminal green fluorescent protein (GFP)-tagged ␣ 1 -ARs showed that ␣ 1A -and ␣ 1B -ARs localize to the plasma membrane, whereas ␣ 1D -ARs accumulate intracellularly. We recently showed that ␣ 1D -and ␣ 1B -ARs form heterodimers, whereas ␣ 1D -and ␣ 1A -ARs do not. Here, we examined the role of heterodimerization in regulating ␣ 1D -AR localization using both confocal imaging of GFP-or CFPtagged ␣ 1 -ARs and a luminometer-based surface expression assay in HEK293 cells. Co-expression with ␣ 1B -ARs caused ␣ 1D -ARs to quantitatively translocate to the cell surface, but co-expression with ␣ 1A -ARs did not. Truncation of the ␣ 1B -AR extracellular N terminus or intracellular C terminus had no effect on surface expression of ␣ 1D -ARs, suggesting primary involvement of the hydrophobic core. Co-transfection with an uncoupled mutant ␣ 1B -AR (⌬12␣ 1B ) increased both ␣ 1D -AR surface expression and coupling to norepinephrine-stimulated Ca 2؉ mobilization. Finally, GFP-tagged ␣ 1D -ARs were not detected on the cell surface when expressed in rat aortic smooth muscle cells that express no endogenous ARs, but were almost exclusively localized on the surface when expressed in DDT 1 MF-2 cells, which express endogenous ␣ 1B -ARs. These studies demonstrate that ␣ 1B /␣ 1D -AR heterodimerization controls surface expression and functional coupling of ␣ 1D -ARs, the N-and Cterminal domains are not involved in this interaction, and that ␣ 1B -AR G protein coupling is not required. These observations may be relevant to many other Class I G protein-coupled receptors, where the functional consequences of heterodimerization are still poorly understood.

Antagonist binding to the beta-adrenergic receptor is largely entropy driven, with only a small enthalpy component. The binding of agonists, on the other hand, is associated with a large decrease in enthalpy which permits a highly unfavourable decrease in entropy. The thermodynamic differences between the binding of agonists and antagonists may provide new insights into the molecular basis for hormone stimulation of adenylate cyclase activity.